11.2A Folds, Faults, and Mountains

11.2A Folds, Faults, and Mountains Folds and Faults

Folds • Over millions of years, stress forces can bend rock like a ribbon or soft dough. • Steady pressures of stress over long periods of time affect sedimentary layers and can fold them into dramatic forms.

Folds Folds : • During mountain building, compressional stresses often bend flat-lying sedimentary rocks into wavelike ripples called folds. • Folds of sedimentary strata come in three main types • Anticlines • Synclines • Monoclines

Anticlines and Synclines Anticlines and Synclines : • An anticline is usually formed by the upfolding, or arching of rock layers. • Often found in association with anticlines are downfolds, or troughs, called synclines. • The anticlines are the folds that go upwards and the synclines are the folds that go downward.

Dips • The angle that a fold or fault makes with the horizontal is called the dip of the fold or fault. • The more the bend in the fold or fault, the stronger the dip. • In the figure at right, folds, faults and dips are visible in B. • In C, the folds are starting to overturn and D and E the folds have overturned all the way and folded over completely.

Monoclines Monoclines • Folds are generally closely related to faults in the Earth’s crust. Examples of this close association can be found in monoclines. • Monoclinesare large step-like folds in otherwise horizontal sedimentary layers. • Monoclines occur as sedimentary layers get folded over a large faulting-block of underlying rock. • Monoclines are a prominent feature of the Colorado Plateau region.

Hanging walls and footwalls Faults • Recall that faults are fractures in the Earth’s crust along which movement has taken place. • The rock surface immediately above the fault is called the hanging wall. • The rock surface below the fault is called the footwall.

Types of Faults Faults • The major types of faults are • Normal faults • Reverse faults • Thrust faults • Strike-slip faults

Types of Faults Faults • Normal faults occur due to tensional stress andreverseandthrust faults occur due to compressional stress. • Compressional forces generally produce foldsas well as faults, resulting in a thickening and shortening of rocks. • Shearing stresses produce strike-slip faults. • Faults are classified according to the type of movement that occurs along the fault.

Normal Faults Normal Faults • A normal fault occurs when the hanging wall block moves down relative to the footwall block. • Most normal faults have steep dips of about 60 degrees. These dips often flatten out with depth. • The movement in normal faults is mainly in a vertical direction, up-down, with some horizontal movement as well. • Because of the slide down of the hanging wall block, normal faults result in the lengthening, or stretching, of the crust. Tensional stress pulls the blocks apart and lets the hanging wall drop downward

Reverse Faults Reverse Faults: • A reverse fault is a fault in which the hanging block moves up (instead of down) relative to the footwall block. • Reverse faults are high angle compressionalfaults with dips greater than 45 degrees.

Thrust Faults Thrust Faults: • Thrust faults are reverse faults with dips of less than 45 degrees. • Because the hanging wall block moves up and over the footwall block, reverse and thrust faults result in a compression, squeezing and shortening, of the crust.

Thrust Faults Thrust Faults: • Most high-angle reverse faults are small in scale. They cause only local displacements in regions that are already filled with other types of faulting. • Thrust faults, however, exist at all scales. Many can be quite large. • In the Swiss Alps, the northern Rockies, Himalayas, and Appalachians, thrust faults have displaced layers as far as 50 kilometers. • The result of this type of movement is that older rocks end up on top of younger rocks.

Strike-Slip Faults Strike-Slip Faults: • Faults in which the movement is horizontal and parallel to the line of the fault is called a strike-slip fault. • Because of their large scale, and linear nature ( in a line) many strike-slip faults produce a trace that can be seen over a great distance. • Rather than a single fracture, large strike-slip faults usually consist of a zone of roughly parallel fractures.

Strike-Slip Faults Fence break created by strike-slip fault Strike-Slip Faults: • The zone of parallel fractures created by a strike-slip fault may be up to several kilometers wide. • The most recent movement is often along a section only a few meters wide and may offset features such as stream channels. • Crushed and broken rocks produced during faulting are more easily eroded, often producing linear valleys or troughs that mark strike-slip faults.

11.2B Folds, Faults, and Mountains Mountains, Plateaus, Domes and Basins

Types of Mountains • Folding and faulting produce many but not all of Earth’s mountains. • In general, mountains are classified by the processes that formed them • The major types of mountain types include • Volcanic mountains • Folded mountains • Fault-block mountains • Dome mountains

Mountain Ranges Types of Mountains : • Geologists refer to the collection of processes involved in mountain building as orogenesis.The term is derived from the Greek orosmeaning “mountain” and the –genymeaning “born”. • Earth’s mountains do not occur at random. Several mountains of similar shape, age, size and structure form a group called a mountain range.

Mountain Systems Types of Mountains : • A group of different mountain ranges in the same region form a mountain system. • The Sangre de Cristo and West Elk mountain ranges form part of the Rocky Mountain system. Sangre de Cristo Mountains Range Rocky Mountain System

Volcanic Mountains • Recall from the previous chapters that volcanic mountains form along plate boundaries and at hot spots. • In addition, igneous activity forms rock deep in the crust that can be uplifted as a result of plate motions and isostatic adjustment.

Folded Mountains • Mountains that are formed primarily by folding are called folded mountains. • Compressional stress is the major cause of folded mountains. • Compressional stress helped to form the Alps in Europe. • Thrust faulting is also important in the formation of folded mountains, which are often called fold-and-thrust belts.

Folded Mountains • Folded mountains often contain numerous stacked thrust faults that have displaced the folded rocks layers many kilometers horizontally. • The Appalachian Mountains, the northern Rocky Mountains, and the Alps in Europe are all examples of folded mountain ranges. Stacked thrust faults

Fault-Block Mountains • Fault block mountains; another type of mountain formation, is the result of movement along normal faults. • Most normal faults are small and have displacements of only a meter or so. • Others extend for tens of kilometers where they may outline the boundary of a mountain front. Examples fault block mountains

Fault-Block Mountains • Large scale normal faults are associated with fault-block mountains • Fault-block mountains form as large blocks of crust are uplifted and tilted along normal faults. Examples fault block mountains

Grabens and Horsts • Normal faulting occurs where tensional stresses cause the crust to be stretched or extended. • As the crust is stretched, a block called a graben, which is bounded by normal faults, drops down. • Grabens produce an elongated valley bordered by relatively uplifted structures called horsts.

Grabens and Horsts • The Basin and Range regions of Nevada, Utah, and California is made of elongated grabens. • Above the grabens, tilted fault-blocks or horsts produce parallel rows of fault-block mountains. Sierra Nevada Range

Grabens and Horsts • In the western US, other examples of fault block mountains include the Grand Tetons and the Sierra Nevada Range in California. • These steep mountain fronts were produced over 5 to 10 million years by many episodes of faulting. Sierra Nevada Range

Plateaus, domes, basins • Mountains are not the only landforms that result from forces in Earth’s crust. • Up and down movements of the crust can produce a variety of landforms, including • plateaus • domes • basins.

Plateaus • A plateauis a landform with a relatively high elevation and more or less level surface. • To form a plateau, a broad area of the crust is uplifted vertically; raised above the adjoining landscape. • Plateaus can cover very large areas of land such as the Colorado Plateau which stretches over four states. Colorado Plateau

Domes • Broad upwarping in the rock underlying an area may deform sedimentary layers. • When upwarping produces a roughly circular structure, the feature is called a dome. • Domes often have the shape of an elongated oval. • You can think of the upwarpedlayers that make up a dome as a large fold.

Basins • Downwarped structures that have a roughly circular shape are called basins. • The central United States contains a number of basins, including the large Michigan Basin. Michigan Basin

Basins • During mountain building, plate motions can cause the crust to bend downward and form a basin. • If the basin sinks below sea level, it may form a shallow sea. • Over time, sediments such as sand and the skeletons of ocean creatures are laid down, forming layers of sedimentary rock. Michigan Basin

Basins • Basins may also form along the edges of continents where thick layers of sediment build up. The weight of the sediment downwarps the crust to form a basin. • When forces in the crust uplift the sedimentary layers, the rock that fills the basin is exposed at the surface. Michigan Basin

Basins • Look at the map of the Michigan Basin to the right; it resembles a bull’s eye. The oldest rocks are around the edges of the basin and the youngest rocks are near the center. Michigan Basin

Basins • The plate motions that help to form sedimentary basins can also destroy them. • For example, when two continental plates collide, the ocean basin between them closes up. • Sedimentary rock in the basin becomes part of the landmass formed by the collision. Michigan Basin

11.2A Folds, Faults, and Mountains