Deformation: Modification of Rocks by Folding and Fracturing PDF

# Deformation: Modification of Rocks by Folding and Fracturing ## When Rocks Are Caught Up in Plate Boundaries - Rocks experience transformation by metamorphism and deformation. - Deformation includes squeezing, stretching, folding, and faulting. - Deformation can occur on individual rock scale, m...

# Deformation: Modification of Rocks by Folding and Fracturing ## When Rocks Are Caught Up in Plate Boundaries - Rocks experience transformation by metamorphism and deformation. - Deformation includes squeezing, stretching, folding, and faulting. - Deformation can occur on individual rock scale, modifying granite into gneiss and sediment into schist. - It also affects layers of sediments, distorting them into complex patterns. - Early geologists assumed that sedimentary rocks were originally deposited as soft horizontal layers, hardened over time. - The question is, what forces act on these rocks to produce specific deformation patterns? - Discovery of plate tectonics in the 1960s provided the answer. ## Plate Tectonic Forces - **Tensional Forces** occur at divergent boundaries where plates move apart. - **Compressive forces** occur at convergent boundaries where plates move toward each other. - **Shearing forces** occur at transform-fault boundaries where plates slide past each other. - Although ideal plate boundaries are sharp, they are smeared out across hundreds or thousands of kilometers in continental crust. - This is why continental crust deforms through folding and faulting, creating folds like folds in cloth when pressed together and faults when layers break and slip under pressure. - These folds and faults can range in size from centimeters to kilometers, forming mountain ranges. - Analysis of deformation patterns helps deduce the movement of plates in the geologic past and reconstruct the tectonic history of the continental crust. ## Mapping Geologic Structure - Faults and folds are analyzed to reconstruct crustal deformation. - Their orientation is indicated by strike and dip, which are important measurements for geologic maps. - **Strike** refers to the compass direction of a rock layer where it intersects a horizontal surface. - **Dip** is the angle that the layer inclines from the horizontal. - **Geologic maps** are two-dimensional representations of rock formations exposed at Earth's surface, using color to denote different rock types. - **Geologic cross sections** are used to depict the three-dimensional structure of rock layers in a specific region. ## How Rocks Deform - Rocks deform depending on the tectonic forces acting on them. - Deformation can occur through folding or faulting or a combination of both. - Key factors affecting deformation include: - Orientation of forces - Rock type - Physical conditions (temperature and pressure) ## Brittle and Ductile Behavior of Rocks in The Laboratory - Experiments with rock deformation are performed under specific conditions of pressure and temperature to simulate crustal environments. - **Brittle Deformation** under low confining pressure, similar to shallow crustal depths results in rock fracturing. - **Ductile Deformation** under high confining pressure, similar to deep crustal depths results in rock bending and deformation without fracturing. - **Ductile Behavior** can also occur at lower confining pressures when rock is heated to temperatures similar to metamorphic conditions. ## Brittle and Ductile Behavior of Rocks in Earth's Crust - Laboratory conditions cannot fully replicate natural conditions in Earth's crust. - **Brittle Behavior** is common at shallow depths where pressure and temperature are lower. - **Ductile Behavior** is dominant at deep pressures and temperatures, often accompanied by metamorphism. - **Rock Type** also plays a role, with igneous and metamorphic rocks behaving as brittle materials and sedimentary rocks exhibiting ductile behavior. - The rate of deformation can also influence behavior, with fast deformation resulting in brittle behavior and slow deformation resulting in a ductile response. - Rocks are more prone to breaking under tension (pulling and stretching) than under compression (squeezing). ## Basic Deformation Structures ### Faults - A **fault** is a fracture that displaces the rock on either side of it. - The movement of rock is measured by **slip direction** and **offset**. - **Dip-slip faults** show movement up or down the dip of the fault plane: - **Normal faults** occur when the hanging wall moves downward relative to the footwall. - **Reverse faults** occur when the hanging wall moves upward relative to the footwall. - **Thrust faults** are low-angled reverse faults with a dip of less than 45 degrees, resulting in a more horizontal movement. - **Strike-slip faults** show horizontal movement parallel to the strike of the fault plane. - **Left-lateral** faults show movement to the left. - **Right-lateral** faults show movement to the right. - **Oblique-slip faults** involve both strike-slip and dip-slip motion. ### Folds - **Folds** are a common deformation structure in layered rocks. - They are formed when a planar structure, such as a sedimentary bed, bends into a curved shape. - **Anticline** rocks fold upwards, forming arches. - **Syncline** rocks fold downwards, forming troughs. - The two sides of a fold are called **limbs**. - The **axial plane** divides the fold symmetrically, with an **axis** that is the lengthwise intersection with the rock layers. - **Horizontal folds** have a horizontal fold axis and vertical axial plane. - **Plunging folds** have a fold axis at an angle to the horizontal, resulting in a zigzag pattern of outcrops. - Folds can be **symmetrical**, with limbs dipping equally from the axial plane. - Folds can be **asymmetrical**, with one limb dipping more steeply than the other. - **Overturned folds** are extremely asymmetrical folds that have one limb tilted beyond vertical, resulting in both limbs dipping in the same direction. ## Circular Structures - **Basins** are synclinal (downward) structures with rock layers dipping towards the central point. - **Domes** are anticlinal (upward) structures, circular or oval in shape, with rock layers dipping radially away from a central point. - These structures can be caused by: - **Magma upwelling** - **Crustal subsidence** - **Stretching and thinning of the crust** - **Sediment deposition** - **Flexural forces** ## Deformation Textures - **Joints** are cracks in a rock formation along which there is no displacement. - They can be caused by tectonic forces or cooling and contraction of rocks. - **Cataclastic textures** are formed by brittle shearing of rocks, resulting in broken and angular fragments. - **Mylonites** are metamorphic rocks formed by ductile shearing of rocks, resulting in elongated crystals and bands. ## Styles Of Continental Deformation - **Tensional Tectonics** dominates continental crust extension, resulting in **normal faulting** with high dip angles in the brittle upper crust that flatten with depth in the ductile lower crust, creating rift valleys bounded by normal faults. - **Compressions Tectonics** dominates continental crust compression, resulting in **thrust faults** that form **fold and thrust belts**. - **Shearing Tectonics** dominates near-vertical **strike-slip faulting** along major transform faults, with local **compression** at left bends and local **extension** at right bends. ## Unraveling Geologic History - Every geologic region undergoes multiple episodes of deformation and other geologic processes. - Understanding these processes requires reconstructing the geologic history. - **Geologic Maps** and **Cross Sections** are crucial to this process. - They provide evidence of deposition, tilting, faulting, erosion, volcanism, and other events. - By analyzing the order and orientation of rock layers. ## Key Terms and Concepts - **Anticline:** a fold that bends upward. - **Basin:** a downward-shaped depression of rock layers. - **Brittle:** a material that tends to fracture when subjected to stress. - **Compressive forces:** forces that push together or squeeze an object. - **Deformation:** any change in the shape or volume of a rock. - **Dip:** the angle at which a rock layer inclines from the horizontal. - **Dip-slip fault:** a fault in which the movement is up or down the dip of the fault plane. - **Dome:** an upward-shaped bulge of rock layers. - **Ductile:** a material that tends to deform without fracturing when subjected to stress. - **Fault:** a fracture in the Earth's crust along which movement has occurred. - **Fold:** a bend in a rock layer. - **Foot Wall:** the block of rock that lies below a fault plane. - **Formation:** a distinct set of rock layers that can be recognized and mapped as a unit. - **Geologic cross-section:** a diagrammatic representation of a vertical slice through a geologic structure. - **Geologic map:** a two-dimensional representation of the geologic features exposed at the Earth's surface. - **Hanging wall:** the block of rock that lies above a fault plane. - **Joint:** a fracture in rock along with no movement has occurred. - **Normal fault:** a dip-slip fault in which the hanging wall has moved downward relative to the footwall. - **Oblique-slip fault:** a fault that involves both strike-slip and dip-slip motion. - **Shearing force:** a force that causes a rock to slide past one another. - **Strike:** the compass direction of a rock layer where it intersects a horizontal surface. - **Strike-slip fault:** a fault in which the movement is parallel to the strike of the fault plane. - **Syncline:** a fold that bends downward. - **Tensional force:** a force that pulls or stretches an object. - **Thrust fault:** a low-angled reverse fault. ## Practicing Geology Exercise: How Do We Use Geologic Maps to Find Oil? - Petroleum reservoirs are often found in **anticlines**, where it is trapped under an impermeable **cap rock**. - The discovery of oil reservoirs is often based on mapping fold structures and identifying potential traps. - Geologist T. Sterry Hunt was among the first to propose this theory in 1861, revolutionizing oil exploration.