Environmental Engineering Reference
In-Depth Information
Arizona was located along the southwestern edge of the North American continental
core between divergent plate boundaries by the beginning of the Paleozoic Era (550-248
million years ago), but by the end of the Paleozoic, these had become convergent boundaries
during the assembly of the supercontinent of Pangaea. Sedimentary rocks deposited
in Arizona during the Paleozoic record repeating cycles of inundation by shallow seas
alternating with periods of land surface exposure; these cycles can be clearly recognized
in the undeformed and nonmetamorphosed rocks of the Colorado Plateau.*
During the Mesozoic Era (248-65 million years ago), volcanism associated with a
subduction zone to the west produced thick sequences of igneous rocks, both eruptive
and intrusive, throughout much of the southern third of Arizona. Deposition of sediments
derived from the volcanic rocks to the south, as well as sediments derived from source
regions to the north, occurred in other areas of Arizona and indicated a complex variety
of wind- and water-dominated depositional environments during the middle Mesozoic.
The end of the Mesozoic and beginning of the Cenozoic Era (65 million years ago to
the present) is marked by a significant tectonic event known as the Laramide Orogeny.
This event is generally thought to have been caused by atypical shallow subduction of
oceanic crust beneath the North American continent. The shallow subduction caused
widespread volcanism and intrusion of igneous rock throughout much of the southern
half of Arizona, and some areas of northern Arizona on the Colorado Plateau, between
80 and 55 million years ago. The crust was thickened and uplifted during the Laramide
event such that Paleozoic and Mesozoic rocks deposited in central and western Arizona
were subsequently removed by erosion and/or metamorphosed. The eroded material was
transported to the northeast, as southern and western Arizona was a highland during
Laramide time.
Another pulse of volcanism occurred between 37 and 15 million years ago, beginning
in the southeast of Arizona and migrating westward through the southern half of the
state. This volcanic activity is thought to be the result of renewed magma production as
the subducted slab of oceanic crust beneath western North America increased its angle
of descent. The compression of continental crust caused by the shallow slab subduction
was now relaxed, and the crust began to extend and thin. The Colorado Plateau remained
relatively unaffected by either the volcanism or crustal extension and became a highland
during this time period reversing previous drainage patterns.
Basin and Range faulting in southern and western Arizona occurred 15-5 million years
ago and may have been related to establishment of the San Andreas transform boundary
along the continental margin, pulling apart of the crust by upwelling mantle material,
or a combination of both mechanisms. Some volcanic fields were active at this time in
southern and western Arizona. Three large and spatially separate volcanic fields located
along the Transition Zone—Colorado Plateau boundary were also active during Basin
and Range faulting; activity in the San Francisco field near Flagstaff has continued into
historical times. Arizona is relatively quiescent today in terms of plate tectonics because
it is not located on a convergent or divergent plate boundary. The extreme southwestern
part of the state near Yuma, however, is at high risk from earthquakes associated with the
nearby San Andreas transform boundary. The Colorado Plateau region north of Flagstaff
has also experienced frequent moderate earthquakes and thus also has higher earthquake
risk than other parts of the state.
River drainages across the state began to integrate after opening of the Gulf of California
along the San Andreas transform boundary 6-5 million years ago. This is also when the
* For in-depth discussions of the geology of the Grand Canyon and Colorado Plateau, see Beus and Morales. 8
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