Deformation: Modification of Rocks by Folding and Fracturing

# Deformation: Modification of Rocks by Folding and Fracturing ## When Rocks Are Caught Up in Plate Boundaries - Rocks can be transformed by metamorphism. - Deformation is the most important process in regional metamorphism. - Deformation is the modification of rocks by squeezing, stretching, fo...

# Deformation: Modification of Rocks by Folding and Fracturing ## When Rocks Are Caught Up in Plate Boundaries - Rocks can be transformed by metamorphism. - Deformation is the most important process in regional metamorphism. - Deformation is the modification of rocks by squeezing, stretching, folding, and faulting. It occurs on scales ranging from individual rocks to entire continents.  ## Deformation: Modification of Rocks by Folding and Fracturing - The discovery of plate tectonics in the 1960s provided the answers to the questions about how rocks are deformed. - This chapter describes how rocks can be tilted, bent, and fractured. - It focuses on the processes of folding and faulting that deform continental rocks near plate boundaries. ## Plate Tectonic Forces - **Tensional forces** stretch and pull rock formations apart, dominating at divergent boundaries. - **Compressive forces** squeeze and shorten rock formations, dominating at convergent boundaries. - **Shearing forces** shear two parts of a rock formation in opposite directions, dominating at transform fault boundaries. ## Mapping Geologic Structure - **Faults** are fractures that displace rock on either side of them. - **Folds** are bends in rock layers. - **Strike** is the compass direction of a rock layer. - **Dip** is the amount of tilting, the angle at which the rock layer inclines. - **Geologic maps** are two-dimensional representations of Earth's surface geology. ## How Rocks Deform - Rocks deform in response to tectonic forces. - **Brittle deformation** occurs when rocks break and fracture. - **Ductile deformation** occurs when rocks bend without fracturing. ## Basic Deformation Structures ### _Faults_ - **Dip-Slip Faults** relative movement of blocks of rock up or down the dip of the fault plane. - **Normal faults** occur when the hanging wall moves down relative to the foot wall, extending the structure horizontally. - **Reverse faults** occur when the hanging wall moves up relative to the foot wall, shortening the structure. - **Thrust faults** are low-angled reverse faults. - **Strike-Slip Faults** relative movement of blocks is horizontal. - **Oblique-Slip Faults** relative movement of blocks is both horizontal and vertical. ### _Folds_ - Folds occur when an originally planar structure, such as a sedimentary bed, is bent into a curved structure. - **Anticlines** are folds bent upward. - **Synclines** are folds bent downward. - **Plunging folds** occur when a fold's axis is not horizontal. - **Symmetrical folds** have limbs that dip symmetrically. - **Asymmetrical folds** have limbs that dip at different angles. - **Overturned folds** occur when one limb of a fold has been tilted beyond the vertical.   ## Circular Structures - **Basins** are synclinal structures that dip toward a central point. - **Domes** are anticlinal structures that bulge upward. ## Deformation Textures - **Joints** are cracks in a rock formation along which there has been no appreciable movement. - **Cataclastic textures** result when rocks break into angular fragments. - **Mylonites** are metamorphic rocks that have recrystallized due to intense shearing. ## Styles of Continental Deformation - **Tensional tectonics** produces rift valleys and normal faults. - **Compressive tectonics** produces fold and thrust belts, and is characterized by thrust faulting. - **Shearing tectonics** produces strike-slip faults, such as the San Andreas Fault. Transformation faults typically show bends that may cause compression or tension along the fault lines. ## Unraveling Geologic History - Geologists use observations of faults, folds, and other deformation structures to reconstruct the history of a region. ## Practicing Geology Exercise: How Do We Use Geologic Maps to Find Oil? - **T. Sterry Hunt**, a Connecticut-born geochemist, was the first to identify anticlinal structures as the best place to find large petroleum reservoirs. - **Anticlinal traps** occur when porous "reservoir rocks" are overlain by impermeable "cap rocks." - **Oil** is lighter than water and rises toward the surface. - **Anticlines** are excellent locations for oil to accumulate, because the oil cannot escape upward. ## Media Support - Animation videos of strike and dip, reverse and thrust faults, normal faults, strike-slip faults, and rock folding. - An animation video of a dome in the desert: Upheaval Dome. The media support referenced are not provided in the text and would need to be separately accessed.