🌋Physical Geology Unit 9 – Structural Geology and Geologic Maps

Key Concepts and Terminology

• Structural geology studies the deformation of rocks and the resulting geologic structures
• Stress is the force applied to a rock per unit area and can be compressive, tensile, or shear
• Strain measures the change in shape or size of a rock as a result of stress
• Ductile deformation occurs when rocks deform plastically without fracturing (folding)
• Brittle deformation occurs when rocks fracture under stress (faulting)
• Folds are curved or bent layers of rock formed by ductile deformation
  • Anticlines are upward-arching folds
  • Synclines are downward-arching folds
• Faults are fractures in rock along which movement has occurred due to brittle deformation
  • Normal faults form under tensional stress with the hanging wall moving down relative to the footwall
  • Reverse faults form under compressional stress with the hanging wall moving up relative to the footwall
  • Strike-slip faults involve horizontal movement of rock layers past each other

Rock Deformation and Stress

• Rock deformation occurs when stress exceeds the rock's strength causing it to change shape or volume
• Stress can be represented by a stress ellipsoid with three principal axes: σ1 (maximum), σ2 (intermediate), and σ3 (minimum)
• The orientation of the stress field determines the type of deformation (ductile or brittle) and resulting structures
• Confining pressure increases with depth in the Earth and influences the behavior of rocks under stress
• Temperature also affects rock deformation with higher temperatures promoting ductile behavior
• Strain rate the speed at which deformation occurs impacts the deformation style (ductile vs. brittle)
• Elastic deformation is reversible deformation where the rock returns to its original shape when stress is removed
• Plastic deformation is irreversible deformation where the rock permanently changes shape without fracturing

Types of Geologic Structures

• Folds are formed by ductile deformation and can be classified based on their geometry and orientation
  • Monoclines are step-like folds with one limb dipping more steeply than the other
  • Plunging folds have fold axes that are inclined relative to horizontal
  • Recumbent folds have fold limbs that are nearly horizontal
• Domes and basins are circular or elliptical structures formed by upwarping (domes) or downwarping (basins) of rock layers
• Unconformities represent gaps in the geologic record due to erosion or non-deposition
  • Angular unconformities form when tilted or folded strata are overlain by younger horizontal strata
  • Disconformities are erosional surfaces between parallel strata
  • Nonconformities occur when sedimentary rocks overlie igneous or metamorphic rocks
• Joints are fractures in rock along which no significant displacement has occurred
• Foliations are planar features in metamorphic rocks formed by the alignment of minerals during deformation

Reading and Interpreting Geologic Maps

• Geologic maps depict the distribution, age, and relationships of rock units and geologic structures in an area
• Stratigraphic units are represented by different colors or patterns on the map
• Contacts between rock units are shown as lines with specific symbols indicating the type of contact
• Strike and dip symbols indicate the orientation of rock layers or planar features
  • Strike is the compass direction of a horizontal line on a tilted plane
  • Dip is the angle between the tilted plane and horizontal measured perpendicular to strike
• Cross-sections are vertical slices through the Earth that show subsurface relationships between rock units and structures
• Map scale relates distance on the map to actual distance on the ground
• Topographic contours represent lines of equal elevation and provide information about the shape of the land surface
• Geologic map symbols include those for faults, folds, and other structures

Field Techniques and Data Collection

• Field observations and measurements are essential for creating accurate geologic maps and interpreting geologic structures
• Measuring strike and dip using a compass and clinometer to determine the orientation of rock layers and planar features
• Describing rock types, textures, and mineralogy in the field to identify and differentiate rock units
• Collecting structural data such as the orientation of folds, faults, and joints using standard notation
• Creating field sketches and diagrams to illustrate relationships between rock units and structures
• Using GPS to record the location of important geologic features and contacts
• Collecting samples for laboratory analysis (thin sections, geochemistry) to supplement field observations
• Measuring stratigraphic sections to determine the thickness and sequence of rock units
• Interpreting geologic structures in the field based on observable features and relationships

Structural Analysis Methods

• Stereographic projection is a technique for representing 3D orientation data on a 2D surface using a stereonet
  • Planar features are plotted as great circles or poles on the stereonet
  • Linear features are plotted as points on the stereonet
• Equal area and equal angle stereonets are two common types used in structural analysis
• Stereonet analysis can be used to determine the orientation of fold axes, fault planes, and other structures
• Cross-section construction involves projecting surface data into the subsurface using strike and dip measurements
• Balanced cross-sections maintain constant bed thickness and area during deformation
• Restoration of cross-sections involves reversing the effects of deformation to reconstruct the original geometry of rock units
• Strain analysis techniques (Fry method, Rf/φ) quantify the amount and orientation of strain in deformed rocks
• Kinematic analysis uses structural data to interpret the direction and sense of movement on faults and shear zones

Applications in Resource Exploration

• Structural geology plays a critical role in the exploration and development of natural resources (oil, gas, minerals)
• Subsurface structural traps (anticlines, faults) are important targets for oil and gas exploration
• Seismic reflection data is used to image subsurface structures and guide drilling operations
• Structural analysis can help predict the location and geometry of ore deposits in mineralized systems
• Understanding the structural history of an area is important for assessing the timing and formation of resource deposits
• Geologic mapping and cross-section construction are used to create 3D models of subsurface resource distribution
• Fracture analysis is important for understanding fluid flow in reservoirs and planning hydraulic fracturing operations
• Structural data is integrated with other geologic and geophysical data to create comprehensive exploration models

Case Studies and Real-World Examples

• The Zagros fold and thrust belt in Iran is a classic example of a collisional orogen with extensive oil and gas reserves
• The Himalayan mountain range formed as a result of the collision between the Indian and Eurasian plates
• The San Andreas Fault in California is a transform boundary accommodating the relative motion between the Pacific and North American plates
• The Appalachian Mountains in eastern North America formed during multiple orogenic events and contain significant coal and natural gas resources
• The Bushveld Complex in South Africa is a layered igneous intrusion that hosts major deposits of platinum group metals and chromium
• The Athabasca Basin in Canada is a sedimentary basin with unconformity-related uranium deposits
• The North Sea is a prolific oil and gas province with reservoirs in structural traps associated with salt diapirism and extensional faulting
• The Sudbury Basin in Ontario, Canada is an impact structure that hosts significant nickel and copper sulfide deposits