Structural Geology and Geologic Maps

Deformation

•  Deformation refers to all changes in the original form and/or size of a rock body

•  Most crustal deformation occurs along plate margins

•  How rocks deform

•  Rocks subjected to stresses begin to deform by folding, flowing, or fracturing

•  Deformation is strain and occurs in response to stress:

–  Compressional stress (Appalachians)

–  Tensional stress (Basin and Range)

–  Shear stress (San Andreas Fault)

•  How rocks deform

•  General characteristics of rock deformation
–  Elastic deformation – the rock returns to nearly its original size and shape when the stress is removed
–  Once the elastic limit (strength) of a rock is surpassed, it either flows (ductile or plastic deformation) or fractures (brittle deformation)
–  Rock strength is influenced by:
•  Temperature
•  Confining pressure
•  Rock type
•  Time

Brittle vs. Ductile (Plastic) Deformation

Strike and Dip

•  Provides a way of describing the orientation of rock beds, fault planes, or joints

•  Strike is the intersection of a horizontal plane with the bed (a horizontal line measured as a compass direction)

•  Dip is the direction straight down the slope and is normal to the strike (measured from horizontal)

Folds

•  During crustal deformation rocks are often bent into a series of wave-like undulations called folds

•  Most folds result from compressional stresses which shorten and thicken the crust

•  Parts of a fold

–  Limbs – refers to the two sides of a fold
–  Axis – a line drawn down the points of maximum curvature of each layer
–  Axial plane – an imaginary surface that divides a fold symmetrically

•  Common types of folds

•  Anticline – upfolded or arched rock layers (oldest rocks in middle)
•  Syncline – downfolds or troughs of rock layers (youngest rocks in middle)
•  Depending on their orientation, anticlines and synclines can be described as
–  Symmetrical, asymmetrical, recumbent (overturned fold), or plunging

Anticlines and Synclines

Anticlines and Synclines

Chevron Folds

Outcrops of Plunging Folds

Outcrops of Plunging Folds

•  Common types of folds

•  Monoclines – large, step-like folds in otherwise horizontal sedimentary strata

•  Other types of folds

•  Dome
–  Upwarped displacement of rocks
–  Circular or slightly elongated structure
–  Oldest rocks in center, younger rocks on the flanks

Monocline

Monoclines May Result From Buried Faults

A Domal Structure (Black Hills of S. Dak.)

•  Other types of folds

•  Basin
–  Circular or slightly elongated structure
–  Downwarped displacement of rocks
–  Youngest rocks are found near the center, oldest rocks on the flanks

Faults

•  Faults are fractures in rocks along which appreciable displacement has taken place

•  Sudden movements (failures) along faults are the cause of most earthquakes

•  Classified by their movement relative to the break (fault plane), and can be parallel to

•  Strike-slip, dip-slip, or oblique-slip

•  Types of faults

•  Dip-slip faults

–  Movement is mainly parallel to the dip of the fault surface

–  May produce long, low cliffs called fault scarps

–  Parts of a dip-slip fault include the hanging wall (rock surface above the fault) and the footwall (rock surface below the fault)

Hanging Wall and Footwall for a Fault

•  Types of dip-slip faults

–  Normal fault

•  Hanging wall block moves down relative to the footwall block

•  Accommodate lengthening or extension of the crust

•  Most are small with displacements of a meter or so

•  Larger scale normal faults are associated with structures called fault-block mountains

A Normal Fault

Basin and Range Faulting (Tension)

•  Types of dip-slip faults

–  Reverse and thrust faults

•  Hanging wall block moves up relative to the footwall block

•  Reverse faults have dips greater than 45o and thrust faults have dips less then 45o

•  Accommodate shortening of the crust

•  Strong compressional forces

A Reverse Fault

A Thrust Fault

•  Strike-slip fault

•  Dominant displacement is horizontal and parallel to the strike of the fault
•  Types of strike-slip faults

–  Right-lateral – as you face the fault, the block on the opposite side of the fault moves to the right

–  Left-lateral – as you face the fault, the block on the opposite side of the fault moves to the left

A Strike-Slip Fault

•  Strike-slip fault

•  Transform fault

–  Large strike-slip fault that cuts through the lithosphere

–  Accommodates motion between two large crustal plates

The San Andreas (Transorm) Fault system

Review of Fault Types

Joints

•  Joints are among the most common rock structure

•  A joint is a fracture with no offset

•  Most occur in roughly parallel groups

•  Significance of joints

•  Groundwater movement is often controlled by joint systems

•  A joint system can influence the direction that a stream course follows

•  Chemical weathering tends to be concentrated along joints

Joint Formation

Mountain belts

•  Orogenesis – the processes that col-lectively produce a mountain belt

•  Includes folding, thrust faulting, meta-morphism, and igneous activity

•  Mountain building has occurred during the recent geologic past

•  Alpine-Himalayan chain

•  American Cordillera

•  Mountainous terrains of the western Pacific

Earth’s Major Mountain Belts

•  Older Paleozoic- and Precambrian-age mountains

•  Appalachians

•  Urals in Russia

•  Several hypotheses have been proposed for the formations of Earth’s mountain belts

Mountain building at convergent boundaries

•  Plate tectonics provides a model for orogenesis

•  Mountain building occurs at convergent plate boundaries

•  Of particular interest are active subduction zones

–  Volcanic arcs are typified by the Aleutian Islands and the Andean arc of western South America

•  Aleutian-type mountain building

•  Where two ocean plates converge and one is subducted beneath the other

•  Volcanic island arcs result from the steady subduction of oceanic lithosphere

–  Most are found in the Pacific (the Ring of Fire)

–  Continued development may produce mountainous topography consisting of igneous and metamorphic rocks

–  Active island arcs include the Mariana, New Hebrides, Tonga, Japan, and Aleutian arcs

Formation of a Volcanic Island Arc

•  Andean-type mountain building

•  Mountain building along continental margins

–  Involves the convergence of an oceanic plate and a plate whose leading edge contains continental crust

–  Exemplified by the Andes Mountains

•  Andean-type mountain building

•  Stages of development - passive margin

–  First stage

–  Continental margin is part of the same plate as the adjoining oceanic crust

–  Deposition of sediment on the continental shelf produces a thick wedge of shallow-water sediments

–  Turbidity currents are deposited sediment on the continental rise and slope

•  Andean-type mountain building

•  Stages of development – active continental margins

–  Subduction zone forms

–  Deformation process begins

–  Convergence of the continental block and the subducting oceanic plate leads to deformation and metamorphism of the continental margin

–  Continental volcanic arc develops

•  Andean-type mountain building

•  Stages of development – active continental margins

–  Accretionary wedge may form

•  Chaotic accumulation of sedimentary rocks and metamorphic rocks with occasional scraps of ocean crust

•  Can become large enough to stand above sea level (California Coast Ranges)

•  Andean-type mountain building

•  Composed of roughly two parallel zones

–  Volcanic arc

•  Develops on the continental block

•  Consists of large intrusive bodies intermixed with high-temperature metamorphic rocks

–  Accretionary wedge

•  Seaward segment

•  Consists of folded, faulted, and metamorphosed sediments and volcanic debris

Orogenesis along an Andean-type subduction zone

•  Andean-type mountain building

•  Sierra Nevada and Coast Ranges

–  One of the best examples of an active Andean-type orogenic belt

–  Subduction of the Pacific Basin under the western edge of the North American plate

–  Sierra Nevada batholith formed as a volcanic arc

–  Coast Ranges formed as acrretionary wedge

•  Continental collisions

•  Two lithospheric plates, both carrying continental crust

•  The Himalayan Mountains are a youthful mountain range formed from the collision of India with the Eurasian plate about 45 million years ago

•  Continental collisions

•  The Himalayan Mountains

–  Spreading center that propelled India northward is still active

–  Similar but older collision occurred when the European continent collided with the Asian continent to produce the Ural mountains

Prior to the collision of India with Eurasia

India’s Approach to Eurasia Over Time

Formation of the Himalayas

Basin and Range Created by Tension

Possible Tensional Explanation

•  Continental accretion and mountain building

•  A third mechanism of orogenesis

•  Small crustal fragments collide and merge with continental margins

•  Responsible for many of the mountainous regions rimming the Pacific

•  Accreted crustal blocks are called terranes

•  Continental accretion and mountain building

•  Terranes consist of any crustal fragments whose geologic history is distinct from that of the adjoining terranes

•  As oceanic plates move, they carry embedded oceanic plateaus, volcanic island arcs and microcontinents to an Andean-type subduction zone

Exotic Terranes

How Orogenesis Ends

Vertical movements of the crust

•  In addition to the horizontal movements of lithospheric plates, vertical movement also occurs along plate margins as well as the interiors of continents far from plate boundaries

• 

•  Isostatic adjustment

•  Less dense crust floats on top of the denser and deformable rocks of the mantle

•  Concept of floating crust in gravitational balance is called isostasy

•  If weight is added or removed from the crust, isostatic adjustment will take place as the crust subsides or rebounds

The Principle of Isostasy

Key Terms Chapter 9

Stress and Strain

Pressure

Compression, tension, shear

Elastic, brittle, plastic (ductile) deformation

Craton

Orogen

Isostasy

Structural geology

Strike and dip

Normal, reverse, thrust, strike-slip faults

Folds (limbs, axes)

Anticline, syncline, monocline

Geologic and topographic maps

Geologic cross section