Geologic Structures & Mountain Building

Transcription

1 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.

2 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: Factors Influencing Strain» flows (ductile deformation) or» fractures (brittle deformation). 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.

3 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)

4 Here is an illustration of strike and dip from the laboratory manual. 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. 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.

5 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

6 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)

7 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.

8 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.» Accommodate lengthening or extension of the crust» Most are small with displacements of 1 meter or so.

9 » Larger scale normal faults are associated with structures called fault-block mountains. Important note: Normal faults are formed by tensional stress AND tensional stress is common in divergent plate environments- net effect is to lengthen and thin lithosphere. Horst and Grabens from Normal Faults Horst (aka range) This is the upthrown (higher) block between two normal faults. Graben (aka basin) This is the downthrown (lower) block between two normal faults. Horsts and Grabens = Basin & Range in the Western United Satates. In Phoenix, we live in what s called the Basin and Range Province. The mountains around Phoenix are the ranges or horsts, and the valleys are basins or horsts. Detachment Fault - A normal fault that dips less than 45 degrees. Horst & Half Grabens are also created from a series of detachment faults. Reverse and Thrust faults» The hanging wall block moves up relative to the footwall block.» Reverse faults have dips greater than 45 degrees and thrust faults have dips less then 45 degrees.» Accommodate shortening of the crust» Strong compressional forces Important note: Reverse faults are formed by compressional stress AND compressional stress is common in convergent plate environments- net effect is to shorten and thicken lithosphere. Thrust Fault = Low Angle Reverse Fault. This is a reverse fault dipping less than 45

10 Strike-Slip Faults Dominant displacement is horizontal and parallel to the strike of the fault Types of strike-slip faults Right-lateral (dextral) As you face the fault, the opposite side of the fault moves to the right. Left-lateral (sinistral) As you face the fault, the opposite side of the fault moves to the left. Transform faults a special type of strike slip fault Large strike-slip faults that cut through the lithosphere Accommodate motion between two large crustal plates Important note: strike-slip faults are formed by shear stress AND shear stress is common along transform faults. There is no net shortening or lengthening of the lithosphere. The San Andreas Fault System is a good example of a right lateral transform fault system. Strike slip movements happen over a long period of time.. In short jerks causing earthquakes. In between these movements there is little movement while strain builds. Oblique-slip faults combination of strike slip and dip slip Most faults have some oblique component Most faults tend to be dominantly dip or strike- slip We simply classify most faults as if they were completely one way or the other Special Note: Remember..Fault motion causes earthquakes- NOT vice versa. Now that we have covered the basic structures that are found in mountains and valleys, it s time to look at how mountains are built. Several hypotheses have been proposed for the formations of Earth s mountain belts. With the development of plate tectonics, it appears that most mountain building occurs at convergent plate boundaries. Mountain building has occurred during the recent geologic past. Here are some examples.

11 American Cordillera the western margin of the Americas from Cape Horn to Alaska, which includes the Andes and Rocky Mountains The Alpine Himalaya chain The mountainous terrains of the western Pacific Here are some examples of older Paleozoic and Precambrian-age mountains The Appalachians The Urals in Russia Orogenesis is the processes that collectively produces a mountain belt. Includes folding, thrust faulting, metamorphism, and igneous activity Mountains created by Continental Collisions Fault Block Mountains Two lithospheric plates, both carrying continental crust collide Continental collisions result in the development of compressional mountains that are characterized by shortened and thickened crust. Most compressional mountains exhibit a region of intense folding and thrust faulting called a fold-and-thrust belt. The zone where the two continents collide is called the suture. The Appalachian Mountains older mountain belt Formed long ago and substantially lowered by erosion Resulted from a collision among North America, Europe, and northern Africa Final orogeny occurred about 250 million to 300 million years ago. The Himalayan Mountains younger mountain belt Collision began about 45 million years ago. India collided with Eurasian plate. Similar but older collision occurred when the European continent collided with the Asian continent to produce the Ural Mountains.

12 Formed by Tensional Stress Fault-Block Mountains formed by tensional stress Continental rifting causes the formation of fault-block mountains. Fault-block mountains are bounded by high-angle normal faults. These faults flatten with depth. Examples include the Grand Tetons of Wyoming and Basin and Range province of the western United States. Basin and Range Province One of the largest regions of fault-block mountains on Earth Tilting of these faulted structures has produced nearly parallel mountain ranges that average 80 kilometers in length. Extension beginning 20 million years ago has stretched the crust twice its original width. High heat flow and several episodes of volcanism provide evidence that mantle upwelling caused doming of the crust and subsequent extension. Mountain Building created by Plate Convergence and Subducting Plates Major features of subduction zones Deep-ocean trench a region where subducting oceanic lithosphere bends and descends into the asthenosphere Volcanic arc built upon the overlying plate Island arc if on the ocean floor or Continental volcanic arc if oceanic lithosphere is subducted beneath a continental block Subduction and Mountain Building Island arc mountain building Where two ocean plates converge and one is subducted beneath the other

13 Volcanic island arcs result from the steady subduction of oceanic lithosphere. Continued development can result in the formation of mountainous topography consisting of igneous and metamorphic rocks. Mountain Belts- Volcanic on Continents Andean-type mountain building Mountain building along continental margins Involves the convergence of an oceanic plate and a plate whose leading edge contains continental crust Exemplified by the Andes Mountains (best example) Building a volcanic arc Andean Type (brief explanation of how volcanoes form in this setting) Subduction and partial melting of mantle rock generate primary magmas. Magma is less dense than surrounding rock, so it begins to buoyantly rise. Differentiation of magma produces andesitic volcanism dominated by pyroclastics and lavas. Emplacement of plutons Andean Type (brief explanation of how magma creates igneous rock bodies at depth (plutons) and and erosion forms batholiths in this setting) Thick continental crust impedes the ascent of magma. A large percentage of the magma never reaches the surface and is emplaced as plutons. Uplift and erosion exposes these massive structures called batholiths (i.e., the Sierra Nevada in California and the Peruvian Andes) Batholiths are typically intermediate to felsic compositions.

14 Development of an accretionary wedge Andean Type (brief explanation of the formation of a wedge in this setting through the accumulation of rocks carried in and deformed by the subducting plate) An accretionary wedge is a chaotic accumulation of deformed and thrust-faulted sediments and scraps of oceanic crust. Prolonged subduction may thicken an accretionary wedge enough so that it protrudes above sea level. Descending sediments are metamorphosed into a suite of highpressure, low-temperature minerals. Forearc basin Andean Type (brief explanation of basin formation in this setting) The growing accretionary wedge acts as a barrier to sediment movement from the arc to the trench. This region of relatively undeformed layers of sediment and sedimentary rock is called a forearc basin. The Sierra Nevada and the Coast Ranges Example Andean Type One of the best examples of an active Andean-type orogenic belt Subduction of the Pacific basin under the western edge of the North American plate The Sierra Nevada batholith is a remnant of a portion of the continental volcanic arc. The Franciscan Formation of California s Coast Ranges constitutes the accretionary wedge. Terranes and Mountain Building Another mechanism of orogenesis - The nature of terranes (different from Terrain-relief or topography) Small crustal fragments collide and merge with continental margins. Accreted crustal blocks are called terranes (any crustal fragments whose geologic history is distinct from that of the adjoining terranes). Prior to accretion, some of the fragments may have been microcontinents.

15 Others may have been island arcs, submerged crustal fragments, extinct volcanic islands, or submerged oceanic plateaus. Accretion and orogenesis As oceanic plates move, they carry embedded oceanic plateaus, island arcs, and microcontinents to Andean-type subduction zones. Thick oceanic plates carrying oceanic plateaus or lighter igneous rocks of island arcs may be too buoyant to subduct. Accretion in Western U.S. With the breakup of Pangea East Pacific basin (Farallon Plate) subducted under North America Results in addition of crustal fragments to western margin of U.S. Erosional Remnants- Sedona AZ = example In addition to structural mountains, there are also mountains which are formed through erosion. In Sedona, there are numerous flat topped hills/mountains which are formed through the erosion of horizontal sedimentary layers. 8/2011