The Geology of The SouthMountains:

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 SouthMountains, commonly referred to as “SouthMountain”, lie in the Valley of the Sun in Phoenix, Arizona. The Pima Indians (Akimel O’odham) first named these peaks “Muhadag Du’ag”, or “GreasyMountain” because of the dark varnish-like sheen from the igneous rocks. To many of the people who live in “the Valley”, SouthMountain is a land-feature 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 SouthMountain has named are: Main Ridge (with the communication towers); AltaRidge (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 SouthMountain taken on September 23, 1996 from the Space Shuttle Atlantis. Notice that SouthMountain is to the east of Sierra Estrella mountain chain (Courtesy of the NASAJohnsonSpaceCenter):

What is a Metamorphic Core Complex? When Did This Happen?

These rounded, dome-shaped SouthMountains 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 / EPOCH / Millions of Years Ago / Tectonic Events in Arizona
Cenozoic / Quaternary / Holocene
Pleistocene / 0.02
1.8
5
25
37
55
65
135
180
230
275
330
355
410
430
500
600
1000
1500
2000
3000
4500
Tertiary / Pliocene
Miocene
Oligocene
Eocene
Paleocene / Basin and Range Crustal
Extension and Volcanism
Mid-Tertiary Orogeny
Mesozoic / Cretaceous
Jurassic
Triassic / Laramide Orogeny
Marine Transgression
Plutonism and Volcanism
Paleozoic / Permian
Pennsylvanian
Mississippian
Devonian
Silurian
Ordivician
Cambrian / Several Marine Regessions
and Transgressions
Regional Uplift and
Erosion, Regression
Precambrian /
Mazatzal Orogeny and
Plutonism

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 SouthMountain and the WhiteTankMountains. The broad, arched profile of SouthMountain, 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. CamelbackMountain, SquawPeak, etc.) (Allen, 2003; Rystrom, 2004).

Above: Figure 1 (Process of MCC)

Above: Figure 2 (Stretched rock from MCC on SouthMountain)

What Kind of Rock is SouthMountain?

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 SouthMountain 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 SouthMountain

(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 SouthMountain 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. September 25, 2004