Geologic Structures Changes in the shape and/or orientation of rocks in response to applied stress
Figure 15.19 Can be as big as a breadbox
Or much bigger than a breadbox
Three basic types Fractures >>> The rocks break but don t move Faults Folds >>> The rocks and move >>> Rocks don t break, but deform ductiley
What forces are involved?
Stress and Strain Stress is force per unit area The three basic types of stress are compressive, tensional and shear Strain is a change in size or shape in response to stress Geologic structures are indicative of the type of stress and its rate of application, as well the physical properties of the rocks or sediments
Rock Deformation Stress is the pressure or force applied to rocks that cause deformation to occur Uniform (confining) stress is equal in all directions (hydrostatic) Rocks are confined by the rock around them Differential stress is not equal in all directions (directional) This is what deforms rocks
Rock Deformation Three types of differential stress Tensional - pulling apart Compressional - squeezing together Shear - slipping, twisting, or wrenching Strain is the result of applying a stress to a rock The change in size and/or shape of a solid
Tension and compression
Shear stress
Rock Deformation Strain produces a spectrum of deformation Elastic deformation Rocks return to original shape Ductile deformation Irreversible change in size and/or shape Volume and density may change Brittle deformation - Fracture Stress exceeds the ductile limit Irreversible break
How Rocks Respond to Stress Rocks behave as elastic, ductile or brittle materials depending on: amount and rate of stress application type of rock temperature and pressure If deformed materials return to original shape after stress removal, they are behaving elastically However, once the stress exceeds the elastic limit of a rock, it deforms permanently ductile deformation involves bending plastically brittle deformation involves fracturing
Orientation of Geologic Structures Geologic structures are most obvious in deformed sedimentary rocks Tilted beds, joints, and faults are planar features whose orientation is described by their strike and dip Strike is the compass direction of a line formed by the intersection of an inclined plane with a horizontal plane Dip is the direction and angle from horizontal in which a plane is oriented
How do we describe rock relationships in nature?
Geometry of Rock Structures Structures may be defined by the orientation of planes Dip the angle of inclination downward from a horizontal plane Strike the compass bearing of a horizontal line where the inclined plane intersects an imaginary horizontal plane
Figure 15.7
Fig. 7.5. Strike & Dip
Structures and Geologic Maps Rock structures are determined on the ground by geologists observing rock outcrops Outcrops are places where bedrock is exposed at the surface Geologic maps use standardized symbols and patterns to represent rock types and geologic structures, such as tilted beds, joints, faults and folds
Figure 15.8
Figure 15.9
Geologic Structures: Fractures and Faults
Joints Fractures created by tension in brittle rocks No shear or displacement has occurred Form as overburden is removed, confining stress reduced Form by cooling of igneous rocks Often occur in sets
Joint systems
Faults Fractures that have been displaced Most faults are inclined at some angle measured from horizontal The dip angle of the fault Two blocks are defined, one on either side of the fault
Faults Fault geometry Imagine a horizontal tunnel cutting through a fault in cross-section Horizontal Surface Dip angle Hanging Wall Foot Wall Fault plane
Fault Types Faults may be divided into three categories Normal faults Hanging wall moves down relative to foot wall Block slides down the dip angle Reverse faults Hanging wall moves up relative to foot wall Block moves in the reverse direction to what seems normal
Fault Types Strike slip faults Displacement to sideways in a horizontal direction Movement is parallel to the strike of the fault plane Strike is the direction of the line formed by the intersection of the fault plane with the Earth s surface
Major types of faults
Normal Faults Normal faults are created by tensional forces, i.e. pulling apart Rifts are created by parallel normal faults dipping toward each other The block in the center which drops down is a graben The Rio Grande valley in New Mexico is a rift graben
A normal fault?
A special type of normal fault Fault blocks, bounded by normal faults, that drop down or are uplifted are known as grabens and horsts, respectively Grabens associated with divergent plate boundaries are called rifts
Normal faults produce grabens & horsts
Extensional (Normal) Faults reviewed
Reverse Faults Compressional stress usually causes reverse faults to form Reverse faults are common at convergent plate boundaries Reverse faults cause a thickening of the crust as rocks are piled up Older rocks may be found above younger rocks
Reverse Faults Thrust faults are a special kind of reverse fault Shallow dip angle, > 45 o Common in large mountain ranges Horizontal displacement may be many tens of kilometers Evidence of thrust faults in sedimentary rocks is seen when a sequence of the same rocks are repeated
Strike-Slip Faults Strike-Slip faults Principle movement is horizontal Left or Right Lateral Little or no vertical movement Caused by shear stress Indicated by abrupt changes in drainage patterns
Strike-slip faults offset drainage
Types of Faults Strike-slip faults have movement that is predominantly horizontal and parallel to the strike of the fault plane A viewer looking across to the other side of a right-lateral strike-slip fault would observe it to be offset to their right A viewer looking across to the other side of a left-lateral strike-slip fault would observe it to be offset to their left Oblique-slip faults have movement with both vertical and horizontal components Right-lateral San Andreas Fault
F aulitng.exe
Movement Along Faults Rarely exceeds a few meters in a single event Small movements, cm scale, may occur on a regular basis Tectonic creep Total displacement may be km, but does not occur in a single event
Geologic Structures: Folds
Folds Folds are wavelike bends in layered rock Represent rock strained in a ductile manner, usually under compression The axial plane divides a fold into its two limbs The surface trace of an axial plane is called the hinge line (or axis) of the fold Anticlines are upward-arching folds, and synclines are downward-arching folds
Folds Warps in rock strata due to ductile deformation 3-D structures of wide ranging scale Generally indicate horizontal compression Multiple generations of folding may exist
Folds Three simple fold forms exist Synclines warp downward Anticlines warp upward Monoclines dip in one direction
Folds Folds are described by: The strike of their hinge line The hinge line is the intersection of the hinge plane with the folded layer Hinge lines may be inclined in a plunging fold The angle of dip of their limbs
Fold geometry
Types of folds
Anticlines & Synclines The sequence of ages of strata indicate the geologic structure in folds Anticlines have the oldest layers exposed at the center of the fold along the axial plane Synclines have the youngest strata exposed at the center along the axial plane
A series of anticlines & synclines
Fold Belts Orogenic belts consist of long linear series of folds Fold geometry is not overly complex Pattern of outcrops may appear complex Complex folds may develop as folds are: Re-folded Cut by thrust faults
Orogenic belt with complex folding
Complex Folds Folds may be very complex Application of shear stress Multiple folding events Complex forms are created
Complex Folds Plunging folds occur when the folds axis is dipping or plunging Limbs of Asymmetrical folds are not the same, one dips more steeply than the other Overturned and Recumbent folds occur when folding is so extreme that beds are turned upside-down
A plunging anticline
Types of Folds Plunging folds are folds in which the hinge line is not horizontal Where surfaces have been leveled by erosion, plunging folds form V- or horseshoe-shaped patterns of exposed rock layers (beds) Open folds have limbs that dip gently, whereas isoclinal folds have parallel limbs Overturned folds have limbs that dip in the same directions, and recumbent folds are overturned to the point of being horizontal
Folds.exe
Structural Domes and Basins Domes are structures in which the beds dip away from a central point Sometimes called doubly plunging anticlines Basins are structures in which the beds dip toward a central point Sometimes called doubly plunging synclines
Domes & Basins Complex Folds Generally occur in continental interiors Broadly warped regions Roughly circular pattern of outcrops
A small dome
Box 15.1 Figure 1a
Complex Folds Diapirs Less dense salt layers may rise up Some overlying strata may be pierced Salt diapir has an inverted teardrop shape Strata above diapir are domed upward
Mountain Belts and Earth s Systems Mountain belts are chains of mountain ranges that are 1000s of km long Commonly located at or near the edges of continental landmasses Mountain belts are part of the geosphere Form and grow by tectonic and volcanic processes over tens of millions of years As mountains grow higher and steeper, erosion rates (particularly from running water and ice - hydrosphere) increase Air (atmosphere) rising over mountain ranges results in precipitation and erosion
Characteristics of Mountain Belts Mountain belts are very long compared to their width The North American Cordillera runs from southwestern Alaska down to Panama Older mountain ranges (Appalachians) tend to be lower than younger ones (Himalayas) due to erosion Young mountain belts are tens of millions of years old, whereas older ones may be hundreds of millions of years old Ancient mountain belts (billions of years old) have eroded nearly flat to form the stable cores (cratons) of the continents Shields - areas of cratons laid bare by erosion Insert revised Fig. 20.4 h
Rock Patterns in Mountain Belts Mountain belts typically contain thick sequences of folded and faulted sedimentary rocks, often of marine origin May also contain great thicknesses of volcanic rock Fold and thrust belts (composed of many folds and reverse faults) indicate crustal shortening (and thickening) produced by compression Common at convergent boundaries Typically contain large amounts of metamorphic rock
Rock Patterns in Mountain Belts Erosion-resistant batholiths may be left behind as mountain ranges after long periods of erosion Localized tension in uplifting mountain belts can result in normal faulting Horsts and grabens can produce mountains and valleys Earthquakes common along faults in mountain ranges
Evolution of Mountain Belts Rocks (sedimentary and volcanic) that will later be uplifted into mountains are deposited during accumulation stage Typically occurs in marine environment, at opening ocean basin or convergent plate boundary Mountains are uplifted at convergent boundaries during the orogenic stage Result of ocean-continent, arc-continent, or continent-continent convergence Subsequent gravitational collapse and spreading may bring deep-seated rocks to the surface
Evolution of Mountain Belts After convergence stops, a long period of erosion, uplift and block-faulting occurs As erosion removes overlying rock, the crustal root of a mountain range rises by isostatic adjustment Tension in uplifting and spreading crust results in normal faulting and fault-block mountain ranges
Evolution of Mountain Belts Basin-and-Range province of western North America may be the result of delamination Overthickened mantle lithosphere beneath old mountain belt may detach and sink into asthenosphere Resulting inflow of hot asthenosphere can stretch and thin overlying crust, producing normal faults
Growth of Continents Continents grow larger as mountain belts evolve along their margins Accumulation and igneous activity add new continental crust New accreted terranes can be added with each episode of convergence Western North America (especially Alaska) contains many such terranes Numerous terranes, of gradually decreasing age, surround older cratons that form the cores of the continents