The Geology of The South Mountains:
A Metamorphic Core Complex
By:
Brian Sears
900-21-3937
Dr. Larry Middleton
Geology of Arizona
GLG 599
Illustration of Metamorphic Core Complex by Stephen Reynolds, 1996
Picture of mylonite at South Mountain, Phoenix, Arizona: A rock in which there has been differential against one another movement, at a
high enough temperature and pressure such that the mineral grains within it are crushed (Courtesy of Gemland.com)
Introduction of South Mountain:
The South Mountains, commonly referred to as “South Mountain”, lie in the Valley of
the Sun in Phoenix, Arizona. The Pima Indians (Akimel O’odham) first named these peaks
“Muhadag Du’ag”, or “Greasy Mountain” because of the dark varnish-like sheen from the
igneous rocks. To many of the people who live in “the Valley”, South Mountain is a landfeature that sits in south Phoenix and is characterized by the radio and television communication
towers that sit atop it with the red blinking lights (called the Main Ridge). The three main areas
South Mountain has named are: Main Ridge (with the communication towers); Alta Ridge (to the
west of Main Ridge with a high point); and, North Ridge (the lower area southwest of the Main
Ridge, next to the town of Laveen) (Allen, 2003). Below, see picture of South Mountain taken
on September 23, 1996 from the Space Shuttle Atlantis. Notice that South Mountain is to the
east of Sierra Estrella mountain chain (Courtesy of the NASA Johnson Space Center):
What is a Metamorphic Core Complex? When Did This Happen?
These rounded, dome-shaped South Mountains are studied by geologists from all over the
world-it is a geologic feature known as a metamorphic core complex (MCC) where the rocks
were pushed up through the crust of the Earth. South Mountain is studied at length because it is
considered a relatively young metamorphic core complex with rocks that date as young as 8
million years old (see pictures below of types of rocks in the Valley of the Sun and Arizona
Geologic Timeline Chart that support this paragraph) (Allen, 2003; Nations, 1981).
The Rocks of the Valley of the Sun
Arizona Geologic Timeline
ERA
PERIOD
Cenozoic
EPOCH
Quaternary
Holocene
Pleistocene
Tertiary
Pliocene
Miocene
Oligocene
Eocene
Paleocene
Mesozoic
Cretaceous
Jurassic
Triassic
Paleozoic
Permian
Pennsylvanian
Mississippian
Devonian
Silurian
Ordivician
Cambrian
Precambrian
Millions of Years Ago Tectonic Events in Arizona
0.02
1.8
5
25
37
55
65
135
180
230
Basin and Range Crustal
Extension and Volcanism
Mid-Tertiary Orogeny
Laramide Orogeny
Marine Transgression
Plutonism and Volcanism
Several Marine Regessions
and Transgressions
275
330
Regional Uplift and
355
Erosion, Regression
410
430
500
600
1000
1500
Mazatzal Orogeny and
2000
Plutonism
3000
4500
Table adapted from Nations and Stump, 1981
How and Where do Metamorphic Core Complexes Form?
Not all geologists agree on the interpretation of the formation of MCC’s. There are
several theories of metamorphic core complexes that geologists have debated. Some of those
theories are: Crustal Flow; Gravitational Spreading; Isostatic Uplift; Magmatic Underplanting or
Intrusion; Rolling Hinge and Flexural Uplift. What geologists can directly observe is that the
MCC’s seem to have been formed by thermal upwelling within the crust of the Earth,
accompanied by stretching forces that run in this area in a northeast to southwest direction in
Arizona (Allen, 2003).
Notice in the picture below that the black areas indicate metamorphic core complexes in Arizona
The figure above (Reynolds, 1980) details the placement of the MCC’s in Arizona. The general
trend is NW and parallel to the edge of the Colorado Plateau and the Transition Zone.
What do Metamorphic Core Complexes Do?
Metamorphic Core Complexes make up many of the mountain ranges in southern and
western Arizona, including South Mountain and the White Tank Mountains. The broad, arched
profile of South Mountain, for example, displays the domed and elongated structure (see picture
below). The degree of metamorphism increases with depth toward the core. At the surface of
the domed structure of MCC is a detachment fault, where crust displacement has occurred over
many miles (Rystrom, 2004). 25 million years ago, as the Earth continued to pull apart, the crust
thinned out and lighter rocks, which were once very deep, “bobbed up” to the surface like you
see on the surface of South Mountain (see picture below for description[figure 1]). After
millions of years the crust then broke apart perpendicular to the orientation of the stretching (see
picture below [figure 2]), creating faulting. The Valley of the Sun is an example of this and
massive blocks of rocks were forced to stand on end (e.g. Camelback Mountain, Squaw Peak,
etc.) (Allen, 2003; Rystrom, 2004).
Above: Figure 1 (Process of MCC)
Above: Figure 2 (Stretched rock from MCC on South Mountain)
What Kind of Rock is South Mountain?
Rocks within the complex have been metamorphosed (changed by heat and pressure); the
stretched rock is called mylonite. Other morphed rock includes granodiorite of the Tertiary age,
which is formed by an intrusion of molten (igneous) material, in the Earth's crust (see picture on
next page). The granodiorite rock in South Mountain contains at least twice as much plagioclase
as orthoclase (both minerals are types of feldspar) (Allen, 2003).
In foreground is granodiorite forming a peak on South Mountain
(Courtesy of Gemland.com)
Affects of Metamorphic Core Complexes Across North America?
Metamorphic core complex occur throughout the American West, along a trend running
from southern British Columbia into Mexico (shown in the picture below from Rystrom’s
website). In this “belt” of MCC’s, South Mountain is said to be one of the best because it
represents an early phase of the pulling apart of North America -25 million years ago is when the
crust started to stretch in this northeast to southwest direction. In the picture below, Rystrom
maps the MCC’s throughout North America. Geologists argue that South Mountain is one of
many metamorphic core complexes that is stretching the continent in half (Rystrom, 2004).
Picture of Metamorphic Core Complexes across North America
Bibliography
Allen, Richard M. 2003. Website: Gemland.com. September 25, 2004.
Nations, Dale & Edmund Stump. 1981. The Geology of Arizona. Dubuque, Iowa.
Reynolds, Stephen & William Rehrig. 1980. Geologic and geochronologic reconnaissance of a
northwest-trending zone of metamorphic core complexes in southern and western
Arizona. GSA Memoir 153. pgs. 131-158. Boulder, CO.
Rystrom, V.L.. 2004. Website: Arizona MCC.
http://www.colorado.edu/GeolSci/Resources/WUSTectonics/CoreComplex/Arizona.html
September 25, 2004