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Physical Geology Geologic Structures & Mountain Building Many Of Lecture notes - Bill Engstrom: Instructor GLG 101 – Physical Geology Geologic Structures & Mountain Building Many of the sedimentary rocks that we know were deposited horizontally are now tilted. And, some marine sediments are now at high elevations. How do horizontal ocean sediments end up well above sea level? They had to be moved there somehow. In this section we’ll examine the structures we see in rocks as a result of the enormous stresses that occur related to crustal movement and mountain building. First, we need to understand the difference between topography and geologic structures Topography = relief or terrain, the three-dimensional quality of the surface, and the identification of specific landforms. These can be influenced by structures or erosion. In other words, just because you see a hill, it doesn’t mean that the rock is folded beneath it. The hill may have been formed by erosion. Geologic Structures = the three-dimensional distribution of rock units with respect to their deformational histories (What’s beneath the surface) Deformation is a general term that refers to all changes in the original form and/or size of a rock body. Note: Most crustal deformation occurs along plate margins. Deformation involves Force—that which tends to put stationary objects in motion or changes the motions of moving objects • Stress—force applied to a given area (e.g. with the interaction of Earth’s plates) • Types of stress » Compressional (pushing together) stress shortens a rock body. » Tensional (pulling apart) stress tends to elongate or pull apart a rock unit. » Shear (sliding side to side) stress produces a motion similar to slippage that occurs between individual playing cards when the top of the stack is moved relative to the bottom. In what tectonic environments might these stresses occur? These are important to remember. Although knowing the types of stress is OK, what’s really important is understanding where they occur and how it fits into the theory of plate tectonics. Compressional Stress occurs at Convergent Boundaries (e.g. Chile and Japan) Tensional Stress occurs at Divergent Boundaries (e.g. African rift and mid-Atlantic ridge) Shear Stress occurs at Transform Boundaries (e.g. San Andreas Fault) Strain—changes in the shape or size of a rock body ….caused by stress Rocks subjected to stresses greater than their own strength begin to deform by folding, flowing, or fracturing. Types of Deformation – Strain can be elastic or inelastic – Elastic deformation (strain) —The rock returns to nearly its original size and shape when the stress is removed (e.g. like a rubber band) – Inelastic deformation (strain) - Inelastic means that the material or rock does not return to its original state or shape. Once the elastic limit (strength) of a rock is surpassed, it either: » flows (ductile deformation) or » fractures (brittle deformation). Factors Influencing Strain . Strain rate/time - how fast material is deforming . Material/Rock type . Temperature (& confining pressure) conditions Cold materials - exhibit brittle behavior under most stresses Hot materials - behave plastically/ductile. As an example, think of a chocolate bar. If it’s warm in can easily flow/bend/fold. If it’s cold it will fracture/break. Folding (like the warm candy bar) is considered to be inelastic ductile strain, and Faulting (breaking like the cold candy bar) is inelastic brittle strain Folding -Inelastic – ductile deformation under compressive stress During crustal deformation, rocks are often bent into a series of wave-like undulations called folds. Most folds result from compressional stresses that shorten and thicken the crust. How can we describe folds? • Parts of a fold – Limbs refers to the two sides of a fold. – An axis is a line drawn down the points of maximum curvature of each layer. – An axial plane is an imaginary surface that divides a fold symmetrically. We need to be able to map these features. How is that done? Mapping Geologic Structures When conducting a study of a region, a geologist identifies and describes the dominant rock structures. • Usually, only a limited number of outcrops (sites where bedrock is exposed at the surface) are available. • Work is aided by advances in aerial photography, satellite imagery, and global positioning systems (GPSs). Describing and mapping the orientation or attitude (e.g. strike and dip) of a rock layer or fault surface involves determining the features. • Strike (trend) – The compass direction of the line produced by the intersection of an inclined rock layer or fault with a horizontal plane – Generally expressed as an angle relative to north • Dip (inclination) – The angle of inclination of the surface of a rock unit or fault measured from a horizontal plane – Includes both an of inclination and a direction toward which the rock is inclined. Here is an illustration of strike and dip from the laboratory manual. Again……I recommend that you look at the Tutorial on the GCC Geology Home Page – left side Department News & Info - Faculty Home Pages & Contacts - Campus Location - GeoAssist (for help in geology) - Geologic Time, Structures & Maps Tutorial ---- THIS ONE What are the common types of folds? • Anticline—upfolded or arched rock layers (Oldest rock in center & beds dip away from axial plane) • Syncline—downfolds or troughs of rock layers (Youngest rocks in center & beds dip toward axial plane) • Anticlines and synclines may be either…. – Symmetrical, asymmetrical, recumbent (overturned fold), or plunging Here are a couple of ideas to help you remember the difference between Anticlines and Synclines » Anticline = resembles letter “A” or “Anthill” – upturned fold » Syncline = resembles a smile beginning with letter “S”, or “Sink”- downturned fold Folds form topography (hills and mountains). However, when the “solar engine” gets to work, the mountains and hills will erode, eventually to a flat surface). So, we need to look at what the map patterns are after erosion has occurred. Map patterns (e.g. after erosion of surface) are a “mirror” symmetry of rock units (stripes of different units on either side of the fold) • Anticlines – beds dip away from axial plane/fold axis & oldest rock in center • Synclines – beds dip toward axial plane/fold axis & youngest rocks in center Here is an illustration of anticlines and synclines from the laboratory manual Anticlines (and domes) as Petroleum Traps – Economic Significance Oil and gas are less dense than the water that is also trapped in the rock. As a result, oil and gas essentially “float” to the top. Wherever there is an anticline, the oil and gas can be trapped at the top (crest) of the anticlinal flold. Not all anticlines can be observed on the surface of the Earth. Most early petroleum fields were found this way. Now, other methods are used to find subsurface (buried) anticlines in areas where oil and gas might be present. Typically, sound waves are reflected off buried layers of rock to create a “picture” of the subsurface structures (seismic waves). These sound waves are also generated during earthquakes. However, for mapping, devices that “thump” the ground or explosives are used to create the sound source. We will talk about earthquakes and seismic waves in a future lesson. Folds can also be tilted – This is also called “plunging” (not like a plunger you use for a toilet). The rules we covered above remain the same, but now there is an “S” shaped geometry to the map pattern. Plunging folds from the laboratory manual » S-Shaped Geometry – This is the map pattern you would see between a series of folds that occur together. » Age rules/relationships still hold true » Plunge of a fold – Angle the fold axis makes relative to horizontal • Domes (“bulls-eye” map pattern) – Upwarped displacements of rocks – Circular or slightly elongated structures – Oldest rocks are in the center; younger rocks are on the flanks. – All beds dip away from the center The Black Hills of South Dakota is a good example of a large dome. Illustration of domes and basins from the laboratory manual Basins (“bulls-eye” map pattern) – Circular or slightly elongated structures – Downwarped displacements of rocks – Youngest rocks are found near the center; oldest rocks are on the flanks. – All beds dip toward the center The Michigan Basin (all of eastern Michigan) is a good example of a large basin. Monoclines • Large, step-like folds in otherwise horizontal sedimentary strata. • Formed over fault blocks The Black Hills, which is a large dome, is actually flanked (all around the outside edge) by monoclines. From Tarbuck & Lutgens/Pearson Education Folds can form as open or tight folds, and can be overturned. However, there is a limit to how far they can fold. Eventually, the rocks will break. So, let’s look at the features formed by brittle strain (faults and joints). Brittle Structures – Faults & Joints Joints • Very common rock structures • A joint is a fracture with no movement. • Most occur in roughly parallel groups. • Significance of joints • Chemical weathering tends to be concentrated along joints. • Many important mineral deposits are emplaced along joint systems. • Highly jointed rocks often represent a risk to construction projects. Faults • Faults are fractures in rocks, along which appreciable displacement has taken place (movement). • Sudden movements along faults are the cause of most earthquakes. • Classified by their relative movement, which can be horizontal, vertical, or oblique. Fault Types Basic fault types – from the laboratory manual Dip-slip faults – Movement is mainly parallel to the dip of the fault surface – May produce long, low cliffs called fault scarps – Parts of a dip-slip fault include the hanging wall (rock surface above the fault) and the footwall (rock surface below the fault). Hanging Wall & Footwall of a Fault – An important concept (illustration from lab manual) Types of Dip-Slip Faults Normal faults (aka gravity faults) » The hanging wall moves down relative to the footwall.
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