Geologic Structure PDF

# Geologic Structure ## What is a geologic structure? The term probably brings to mind images of folds and faults. The features are formed in response to pushes and pulls associated with the forces that arise from the movement of tectonic plates or as a consequence of differential buoyancy between...

# Geologic Structure ## What is a geologic structure? The term probably brings to mind images of folds and faults. The features are formed in response to pushes and pulls associated with the forces that arise from the movement of tectonic plates or as a consequence of differential buoyancy between parts of Lithosphere. The most fundamental definition of a geologic structure is a geometric feature in rock whose shape, form, and distribution can be described. # Plate Tectonics Plate tectonics is a scientific theory which describes the large-scale motions of Earth's lithosphere. The theory builds on the older concepts of continental drift, developed during the first decades of the 20th century. The lithosphere is broken up into what are called tectonic plates. There are currently seven or eight major, and many minor plates. The lithospheric plates ride on the asthenosphere. These plates move in relation to one another at one of three types of plate boundaries: - Convergent (collisional boundaries) - Divergent (spreading centers) - Conservative transform boundaries. Earthquakes, volcanic activity, mountain-building and oceanic trench formation occur along these plate boundaries. The lateral relative movement of the plates varies, though it is typically 0-100 mm annually. # Structural Geology & Geotectonics: Deformation Nicholas Steno (1631-1686) examined outcrops where the bedding of rock was not horizontal and speculated that strata that do not presently lie in horizontal layers must have in some way been dislocated. Perhaps Steno's establishment of the principle of original horizontality can be viewed as the birth of structural geology. Deformation refers to the structural changes that take place in the original location, orientation, shape, and volume of the body of rock. It includes physical and chemical processes that produce the structural changes. Any body of rock, no matter how strong, will deform if the conditions are right. Structural deformation results from stresses that exceed rock strength. When strength is exceeded, the rock will fail by brittle or ductile deformation, depending on how the physical environment has affected the ability of a rock to withstand stresses which decreases with temperature. The stresses can be created in nature in a number of ways. A figure showing a buckled fence (A) and a broken fence (B) illustrates that fences, like rocks, respond in different ways to shortening. # Structural Geology & Geotectonics: Deformation Contd. Stresses that cause deformation generally build slowly but persistently, but in some situations, high stresses also can generate like in meteor fall. Structural geology can be defined as the study of the architecture of the Earth's crust, resulted from deformation. Structural geology addresses the form, symmetry, geometry, and other structural components of the earth's crust. It also focuses on strength and mechanical properties of crustal materials. Plate tectonics provide an essential backdrop for understanding the significance of structures, especially regional structures. Plate Tectonics forms the basis for understanding the dynamic circumstances producing deformational movement. Plate interactions create the rock-forming processes generating fundamental, original properties of rock assemblages. Continued plate motion generates the stresses that impart to rocks their chief deformational characteristics. # Folds ## Fold Definition - Folds are wave-like structures that result from deformation of bedding, foliation, or other originally planar surfaces in rocks. - Fold can be very broad to tightly compressed. - They may occur as isolated folds or extensive fold trains of different sizes. ## Scale of Folds - Microscopic - Need magnification - Mesoscopic - Occur from hand specimens to outcrop scales - Small parasitic folds have the same shape as equivalent parts of major fold - Macroscopic - Occur on the map scale or larger ## Anatomy of Folds **Image of a fold illustrating key features:** The image shows a fold with the following features: - **Hinge lines (fold axes)** - **Limb** - **Axial planes** - **Axis** - **Hinge line** - **Anticline** - **Syncline** **Image of fold with key features:** The image shows a fold with the following features: - **Axial surface (plane)** - **Crest** - **Hinge** - **Axis** - **Limb** - **Inflection point** - **Trough** **Image of syncline and anticline:** The image shows a syncline and anticline with a fold. The following features are highlighted: - **Hinge line of syncline** - **Hinge line of anticline** - **Youngest rock exposed on surface** - **Oldest rock unit exposed on surface** - **Syncline** - **Anticline** - **Oldest rock unit** **Anatomy of Folds:** - **Hinge Line:** The line joining points of greatest curvature on a folded surface. - **Axial Surface:** When many hinge lines (from multiple folded surfaces) connect forming a plane. - **Plunge:** When the fold hinge is inclined to the horizontal. - **Wavelength:** The distance between subsequent crests or troughs. - **Amplitude:** Half the distance from a crest to the subsequent trough. - **Vergence:** The direction of leaning of the axial surface. - **First-Order Folds:** The master (largest) fold. - **Second-Order Folds:** Smaller folds on the flanks of first-order folds. - **Enveloping Surface:** Enables the relation of small to large scale folds. ## Kinds of Folds - **Anticline:** A fold that is concave towards older rocks in its center. - **Antiform:** A fold that is concave downward. - **Syncline:** A fold that is concave towards younger rocks in its center. - **Synform:** A fold that is concave upward. - **Dome:** An antiform where bedding dips away from the central point. - **Basin:** A synform where bedding dips towards a central point. **Image of kinds of folds**: The image shows various folds, including: - (a) Ages of rocks known: Anticline and Syncline - (b) Ages of rocks unknown: Antiform and Synform - (c) Anticline (doubly plunging) and Syncline (doubly plunging) - (d) Dome and Basin - (e) Antiformal Syncline and Synformal Anticline ## Kinds of Folds - **Cylindrical Folds:** Folds where a cylinder can be inscribed inside the hinge. - **Non-cylindrical Folds:** Folds where the hinge is not parallel. - **Sheath Folds:** Where the fold hinge curves within the axial surface; Normally occur in shear zones. ## Fold Classification **Image of cylindrical folds:** The image shows an illustration of cylindrical folds. **Image of sheath folds:** The image shows an illustration of sheath folds. **Image of noncylindrical fold:** The image shows an illustration of noncylindrical folds. **Other Fold Classifications:** - **Parallel Folds:** Maintain constant layer thickness. - **Concentric Folds:** Parallel folds in which the folded surfaces define circular arcs. - **Ptygmatic Folds:** Have a lobate shape and look like intestines. - **Similar Folds:** Maintain their shape throughout the section and do not die out vertically. - **Chevron and Kink Folds:** Have straight limbs and sharp angular hinges. - **Disharmonic Folds:** Shape or wavelength changes from one layer to another. - **Supratenuous Folds:** The synclines are thickened, and the anticlines are thinned. **Image of different fold types:** The image illustrates different fold types, including: - (a) Parallel-concentric - (b) Parallel - (c) Chevron - (d) Kink - (e) Disharmonic - (f) Supratenuous **Image of folds:** The image illustrates a fold structure showing the following: - Youngest rock - Oldest rock - Anticline - Syncline **Image of symmetrical and asymmetrical folds:** The image illustrates symmetrical and asymmetrical folds with the following highlights: - **Axial plane** - **Anticline** - **Syncline** - **Increasing Intensity of Deformation:** - Symmetrical - Asymetrical - Overturned - Recumbent # Faults - Fractures in rock along which movement has occurred. - Different styles reflect different stresses. **Image of faults:** The image shows a fault in a cross section. **Faults:** Fractures in rocks created by earthquakes. Faults are characterized by the direction of relative movement: slip. - **Dip-slip faults** - Normal - Reverse - **Strike-slip faults** - **Oblique-slip faults** **Terminology:** - **Hanging Wall:** Term used by miners. They could "hang" their light on this side of the fault because it was above them. - **Footwall:** Also from the miners, this side of the wall upon which they could stand below the hanging wall. **Image of different fault types:** The image illustrates various types of faults, including: - **Reverse faulting** - **Normal faulting** - **Strike-slip faulting** **Image of dip-slip faults:** The image illustrates different types of dip-slip faults, including: - (a) Dip-slip fault (normal) - (b) Dip-slip fault (reverse) - (c) Strike-slip fault - (d) Oblique-slip fault **Image of reverse fault:** The image shows a cross section of a reverse fault. **Image of dip-slip fault (reverse):** The image shows a dip-slip fault (reverse) with the following: - **Compression** - **Shortening** **Image of thrust fault:** A cross section of a thrust fault is shown with the following: - **Hanging wall** - **Footwall** - **Cross section** **Image of a keystone thrust fault:** The image illustrates a keystone thrust fault in Southern Nevada. **Image of Strike-slip faults:** The image shows the San Andreas Fault, a strike-slip fault in California. **Image of strike-slip fault:** The image shows a strike-slip fault with the following: - **Shear** **Image of a fold and fault:** The image shows a fold and fault. # Unconformities - Unconformities in sequences of strata represent times of non-deposition and/or erosion that encompass long periods of geologic time, perhaps millions or tens of millions of years. - The rock record is incomplete. - The interval of time not represented by strata is a hiatus. # Types of Unconformities - **Disconformity:** A surface separating younger from older rocks, both of which are parallel to one another. - **Nonconformity:** An erosional surface cut into metamorphic or intrusive rocks and covered by sedimentary rocks. - **Angular unconformity:** An erosional surface on tilted or folded strata over which younger rocks were deposited. **Image of different unconformity types:** The image shows various types of unconformities, including: - Disconfomity - Angular unconformity - Nonconformity - **Unconformities of regional extent may change from one type to another.** - **They may not represent the same amount of geologic time everywhere.** **Image of a disconformity:** The image shows a disconformity between sedimentary rocks in California. **Image of an angular unconformity:** The image shows an angular unconformity in Santa Rosa. **Image of a nonconformity:** The image shows a nonconformity in South Dakota separating Precambrian metamorphic rocks from the overlying Cambrian-aged Deadwood Formation. **Image of a cross-section of earth:** The image shows a cross section of the Earth with different layers and unconformities. **Image of blue tulips:** The image contains two blue tulips with the word “THANKS.”