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Structural Geology The branch of geology that deals with: Form, arrangement and internal architecture of rocks Description, representation, and analysis of structures from the small to moderate scale Reconstruction of the motions of rocks Structural geology provides information about the conditions during regional deformation using structures Main Principles and Concepts Original Horizontality (Steno) Uniformitarianism (James Hutton) Tectonics vs. Structural Geology Both are concerned with the reconstruction of the motions that shape the outer layers of earth Both deal with motion and deformation in the Earth’s crust and upper mantle Tectonic events at all scales produce deformation structures These two disciplines are closely related and interdependent Definitions Tectonics: Study of the origin and geologic evolution (history of motion and deformation) of large areas (regional to global) of the Earth’s lithosphere (e.g., origin of continents; building of mountain belts; formation of ocean floor) Structural Geology: Study of deformation in rocks at scales ranging from submicroscopic to regional (micro-, meso-, and macro-scale) Structural Geologist A geologist who: Studies deformation of rock and Earth’s crust Identifies and interprets geological structures and their tectonic implications Field Tectonic Studies Many tectonic problems are approached by studying structures at outcrop scale, and smaller (microscopic) or larger (100’s to 1000’s of km) scales Systematically observe/record the patterns of rock structures (e.g., fault, fold, foliation, fracture). This gives the geometry of the structures. Next steps Explanation of the structures Kinematics of formation of the structures – motions that occurred in producing them When integrated over a large area, the motions will help to infer the past tectonic motions Use of Models We use geometric, mechanical, and kinematic models to understand deformation on all scales (micro, meso, macro) Geometric model: 3D interpretation of the distribution and orientation of features within the earth crust Kinematic model: Specific history of motion that could have carried the system from an undeformed to its deformed state (or from one configuration to another) Plate tectonic model is a kinematic model Mechanical Model Mechanical model: Based on laws of continuum mechanics Study of rock deformation under applied forces (laboratory work) Model of driving forces of plate tectonic based on the mechanics of convection in the mantle is a mechanical model Analyses Descriptive: Recognize, describe structures by measuring their locations, geometries and orientations Break a structure into structural elements physical & geometric Kinematic: Interprets deformational movements that formed the structures • Translation, Rotation, Distortion, Dilation Dynamic: Interprets forces and stresses from interpreted deformational movements of structures We Study: Changes in the original orientation, location, shape and volume of a rock body (Deformation: changes in shape, position, and/or orientation of a body) Physical and chemical forces that deform rocks Geologic structures that form to accommodate changes Stress - Force applied over an area Strain - Observable deformation in the rock Brittle deformation – e.g., fault: the result of rapidly applied high stress which "break the rock" Ductile deformation – e.g., fold: the result of slowly applied, constant, low stress which "bend the rock" Deformation Includes: strain, rotation, translation Homogeneous strain: strain is the same at every point in the deformed body Undeformed state is used as a reference frame Commonly, we do not know the initial state Coordinate transformation is used to describe a deformation if initial and final states are known Structure A geometric feature in a rock whose shape, form, and distribution can be described Microstructure The small-scale arrangement of geometric and mineralogical elements within a rock Texture Preferred orientation of crystallographic axes in the sample Microfabric Comprises the microstructure and the texture of a material Fundamental Structures Contacts Primary Structures Secondary structures Fractures (Joints, Shear Fractures) Vein - Precipitated minerals from fluid flowing through fractures Fault Fold Fundamental Structures, con’t Foliation - Preferred orientation of planar rock bodies and/or minerals Lineation - Preferred orientation of linear minerals and rocks Shear Zone Zones of deformed rock that have accommodated movement