Faulting, Folding and Mountain

Building

Types of Stress:

1. TensionStress that pulls rock apart

Stress is a force acting on a rock. It has the same units as pressure, but also has a direction. There are three types of stress: tension, compression and shearing.

2. CompressionStress that pushes rock together

3. ShearingStress that causes rocks to slide past one another

Stress

Stress on rock can cause the rock to strain. Strain is a change in shape or size resulting from applied forces (deformation). Rocks only strain when placed under stress. Any rock can be strained. Strain can be elastic, brittle, or ductile.

Stress can happen without strain, but strain cannot happen without stress.

Faults

A fault is a break in the rocks that make up the earth’s crust, along which rocks move.

A break in the earth’s crust that has no movement is called a joint.

2. Foot wall (graben): The underlying block of a fault having an inclined fault plane.

1. Hanging wall (horst): The overlying block of a fault having an inclined fault plane.

Parts of a Fault

Two parts of a fault:

Types of Faults

There are three types of faults:

1. Normal fault2. Reverse fault3. Strike-slip or Lateral fault

Normal Fault

A normal fault occurs when the hanging wall moves downward relative to the footwall.

A normal fault is caused by tension in an area.

Canyonland National Park, Utah

Click each of the pictures to watch a short simulation on

normal faults.

Death Valley, California

Guatemala

Reverse Fault

A reverse fault occurs when the hanging wall moves upward relative to the footwall.

A reverse fault is caused by compression in an area.

Click the picture above to watch a short simulation on reverse faults.

Strike-slip (Lateral) Fault

Las Vegas, Nevada

If the land gets caught as it slides, then an earthquake occurs.

Click the picture above for a short simulation on strike-slip faults.

At a strike-slip fault, the rocks on either side of the fault slide past each other. This sliding force is called shearing. As the plates slide past each other, the forces bend and twist the land.

Folding

Folding occurs in rock layers when rocks bend instead of break.

Two types of folds:

Anticline: upward fold in a rock layerSyncline: downward fold in a rock layer

Compression: anticlines, synclines, reverse faults, thrust faults, folded

mountains, volcanoes, island arcs, trenches

Tension: normal faults, fault block mountains, rift valleys, mid-ocean

ridges, volcanoes

Shearing: strike-slip faults

Stress and Types of Formations Associated with that Stress

Mountain Building

Mountain formation refers to the geological processes that underlie the formation of mountains. These processes are associated with large-scale movements of the earth's crust.

There are three main types of mountains: 1. Volcanic mountains2. Folded mountains3. Fault-block mountains

Volcanic Mountains

Movements of tectonic plates create volcanoes along the plate boundaries which erupt and form mountains. Volcanoes are very common along subduction zones.

Three type of volcanic mountains:

1. Cinder cone2. Shield volcano3. Composite volcano (Stratovolcano)

Volcanoes can also occur at hot spots. Hot spots are weak areas in the earth’s crust where magma can make its way to the surface.

Examples of volcanic mountains include:

• Mount St. Helens in North America • Mount Pinatubo in the Philippines• Mount Kea and Mount Mauna Loa in Hawaii

Mt. St. Helens Mt. Pinatubo

Mt. Mauna Loa

Folded Mountains

When plates collide or undergo subduction (that is ride one over another), the plates might buckle and fold, forming mountains.

Zagros mountain range, as seen from space

Folded mountains are the most common type of mountain. The world’s largest mountain ranges are fold mountains. These ranges were formed over millions of years.

Examples of folded mountains include:• Himalayan Mountains in Asia• the Alps in Europe• the Andes in South America• the Rockies in North America• the Urals in Russia

Himalayan Mountains

Andes Mountains

Alps

Fault-block Mountains

When a fault block is raised or tilted, fault-block mountains can result. Higher blocks are called horsts and troughs are called grabens.

Examples of fault-block mountains include:

• the Sierra Nevada mountains in North America• the Harz Mountains in Germany

Sierra Nevada Mountains

Harz Mountains

Balcones FaultThe Balcones Fault Zone is a fault in Texas that is caused by tension. The Balcones Fault Zone extends from Dallas to the north and Del Rio to the southwest. The West Austin Hill Country is part of a larger geographical area called the Edwards Plateau.

The Balcones Fault zone is made up of many smaller features, including normal faults, grabens, and horsts. One of the obvious features is the Mount Bonnell Fault.

Photo from atop Mt. Bonnell, one of the most prominent geologic features in the Austin area with an elevation of 785 feet.

The location of the fault zone may be related to the Ouachita Mountains, formed 300 million years ago during a continental collision. Although long since eroded away in Texas, the roots of these ancient mountains still exist, buried beneath thousands of feet of sediment. These buried Ouachita Mountains may still be an area of weakness that becomes a preferred site for faulting when stress exists in the Earth's crust.

The Balcones Fault zone was most recently active about 15 million years ago during the Miocene epoch.

The west Austin Hill Country, also known as the Balcones Canyonlands, is composed of rugged topography, with flat-topped hills separated by steep canyons. The Austin Hill Country starts just west of the Balcones Escarpment.

An escarpment is a continuous line of cliffs or steep slopes facing in one general direction which is caused by faults or erosion.

In Austin, the escarpment generally parallels MoPac (Loop 1). To the west of MoPac, Austin becomes hilly, and east of MoPac, Austin is relatively flat. If you drive along Route 360 or RR 2222, you are driving in the west Austin Hill Country.

Faulting occurred in Central Texas millions of years ago, when the coastal plains to the east bent downward while the more stable central Texas interior of the Llano Uplift remained relatively stable. The Balcones Escarpment and the west Austin Hill Country are the result of this fault episode.

The east side of Austin moved downward by as much as 700 feet. After millions of years of erosion, the soft sediments in the Hill Country area were eroded away, exposing the hard, Lower Cretaceous limestones and dolostones. This erosion created hills and steep canyons in the west Austin Hill Country.

Llano Uplift: A roughly oval-shaped area in central Texas where Precambrian- and Paleozoic-aged rocks are structurally high and have been exposed by erosion of the Cretaceous rocks of the Edwards Plateau. These pre-Cretaceous rocks are present in the subsurface throughout much of Texas, but are primarily exposed in the Llano Uplift.

We don’t know the exact age of Balcones faulting, but we are fairly sure that major movement along the fault occurred between 20 and 25 million years ago during the Miocene Period.

Frequently Asked Questions About the Balcones Fault Zone

How old is the faulting in Central Texas?

How long did the faulting last?

The faulting probably lasted several million of years, with approximately 700 to 1,000 feet of uplift occurring in numerous tiny steps.

Probably many earthquakes rocked the Austin area, with movement occurring along the fault, much in the same way that active faults in California create earthquakes today.

Did the faulting create earthquakes?

Can the Balcones fault still move and can it still create earthquakes?

The Balcones fault has not moved in recorded history, but this does not mean that it could not move again! Some think that the fault did move during the early Pleistocene, just over a million years ago. Also, two small earthquakes in the last 130 years (1893 and 1902) may have been caused by small movements along the Balcones fault.

Are the rocks of different ages across the fault?

The rocks near the surface in the west Austin Hill Country are Lower Cretaceous, ranging from 100 to 112 million years in age. They are called the Glen Rose Formation and Edwards Group.

The rocks east of the Balcones Escarpment are Upper Cretaceous rocks, ranging in age from 70 million to 100 million years. The Upper Cretaceous in the Austin area is composed of the Georgetown, Del Rio, Buda, and Eagle Ford Formations, as well as the younger Austin Chalk and Taylor Clay.

How does the west Austin Hill Country affect Austin?

On the positive side, the Austin Hill Country creates beautiful home sites with magnificent views.

The springs that supply water to Austin’s popular Barton Springs swimming pool are controlled by faults. Rainwater in the Barton Creek watershed (a region draining into a body of water) sinks into the ground and becomes groundwater. This water flows through the Edwards Limestone until it hits the big fault at Barton Springs. The groundwater flows up the fault and fills the swimming pool. The average rate of flow from Barton Springs is approximately 53 million gallons per day!

On the negative side, the hill country creates some of the worst flash flooding in the United States. Because of high rainfall rates and rapid runoff in the hill country area, violent floods have hit Austin. Back in 1900, the first Tom Miller Dam was washed away during a great flood, and the lake behind the dam was drained.

Where can I see the effects of faulting?

The Balcones fault is not a single fault but a zone of faults that extend from the escarpment out to the east for approximately 2 to 5 miles (map). Within the zone of faulting, large sections of rocks are not only offset from each other, but they are also fractured (broken) and folded and can be seen in several areas in Austin.

At the intersection of Loop 360 and MoPac an outcrop of Edwards and younger Georgetown limestones (A, B, C) that shows folding caused by movement along the fault. An anticline (beds curve upward) is to the left of the fault, and a syncline (beds curve downward) is to the right of the fault.

A. Edwards and younger Georgetown limestones folded by movement along the fault. Beds of rock on the east side of the fault have moved downward by 10 feet relative to beds on the west side of the fault.

B. An example of an anticline in the Edwards Group at Loop 360 and MoPac

C. An example of a syncline in the Georgetown Member at Loop 360 and MoPac.

Take a look at the rocks at Loop 360 and Bee Caves Road (RR 2244) (D), where a large road cut has exposed a magnificent outcrop of Lower Cretaceous rock. These rocks are flat-lying because they were not in the major zone of faulting. The darker colored rocks at the bottom of the outcrop are the Glen Rose Formation, and the lighter color rocks above are the Edwards Formation.

These rocks are evidence that Austin was periodically submerged under a shallow sea during the Early Cretaceous Period 135 to 102 million years ago. During this time, hundreds of feet of sediment that eventually formed into limestone was deposited in this area. This outcrop exposes approximately 170 feet of rock and represents 1.75 to 3 million years of time. According to these calculations, 1 foot of limestone would have taken between 10,000 and 20,000 years to form. D. A large road cut at the intersection of Loop 360 and Bee Caves Road (2244)

has exposed a magnificent outcrop of Lower Cretaceous rock of the west Austin Hill Country.

Soils east of the Balcones Escarpment are deep, well-developed, and produce excellent farmland. West of the escarpment, soils are thin and rocky and are used as ranchland.

Rocks in the west Austin hill country are Early Cretaceous in age and consist of the Glen Rose and Edwards Formations. They are composed of limestone and dolomite that were deposited in shallow, warm-water seas, which covered much of Texas during this time in Earth's history. Fossils consisting of animals and plants that lived in the seas are abundant in some rock layers. Some of the rocks even have dinosaur footprints!

Fossilized remains of an ancient oyster named Exogyra ponderosa can be found in Austin Group rocks. This provides further evidence that Austin was once covered by a shallow ocean during Cretaceous time more than 100 million years ago.