Chapter 9: Geological Structures and Mountain Building

9.1 Introduction

• Mountains represent one of the most obvious indications of the Earth’s dynamic activity.

• Mountains do not occur in isolation, but as parts of elongate ranges called mountain belts and orogens.

  • Mountain belts: a range of mountains linked together

  • Orogens: mountain building process

• Mountain building: the process of forming a mountain belt.

• Mountain building can lead to

  • Uplift: the vertical rise of the land surface and the rock beneath.

  • Deformation: when they bend, break, or flow.

• Features of deformation are know as geologic structures

  • Joints: Naturally formed cracks in rock.

  • Faults: fractures on which one body of rock slides past another.

  • Folds: a bend or wrinkle of rock layers or foliation; folds form as a consequences of ductile deformation.

  • Foliation: a fabric or layering in rock, a consequence of the alignment of mineral grains.

• Mountain building can lead to metamorphism and igneous activity.

• Orogeny: a mountain building event

• Erosion causes a range to reach sea level, once the peak is gone, metamorphic rocks remain.

9.2 Rock Deformation in the Earth’s Crust

Deformation and Strain

• Rock deformation includes:

  • Bending

  • Breaking

  • Shortening

  • Stretching

  • Shearing

• There are different types of rock deformation:

  • Elastic deformation

  • Brittle deformation

  • Ductile deformation

• During deformation rocks can undergo one or more different changes:

  • A change in location or displacement

  • A change in orientation or rotation

  • A change in shape or distortion

• Strain: the change in shape of an object in response to deformation.

• When a layer becomes longer, stretching has occurred.

• If a layer is shortened, it has undergone shortening.

• Movement of one part of a rock body past another, so that angles between features in the rock change, results in shear strain.

Brittle vs Plastic Deformation

• Brittle deformation: the cracking and fracturing of a material subjected to stress.

• Plastic deformation: process in which mineral grains behave like plastic and become flattened or elongate without cracking or breaking.

• The behavior of a rock depends on factors such as:

  • Temperature

  • Pressure

  • Deformation rate

  • Composition

Force, Stress, and the Causes of Deformation

• Stress: the push, pull, or shear that a material feels when subjected to a force.

• Compression is a type of stress that takes place when a rock is squeezed together.

• Tension occurs when a rock is pulled apart.

• Shear stress develops when one part of a rock body moves sideways past another.

9.3 Brittle Structures

Joints and Veins

• Joints: naturally formed cracks in rocks.

  • Joints are roughly planar structures so their orientation is defined by their strike and dip.

  • They formed in response to tensional stress in brittle rock (rock splits because it was pulled slightly apart).

• Joints may form due to

  • A rock cooling and shrinking

  • A rock undergoes a decrease in pressure

  • A rock layer bends

• Vein: a mineral filled crack.

  • Appear as white stripes cutting across body of rock.

• Strike: the angle between an imaginary horizontal line (the strike line) on the plane and the direction to true north.

• Dip: the angle of the plane’s slope or the angle between a horizontal plane and the dip line (imaginary line parallel to the steepest slope on the structure), as measured in a vertical plane perpendicular to the strike.

• Plunge: the angle between a line and horizontal in the vertical plane that contains the line.

• Bearing: the compass heading the line (the angle between the projection of the line on the horizontal plane and the direction to true north).

Faults: Surfaces of Slip

• Fault: a fracture on which sliding occurs

  • Can cause earthquakes

  • Planar structures

  • Active faults and inactive faults

Fault Classification

• Focuses on two characteristics of faults

  • The dip/slope of the fault surface

    • Dip can be vertical, horizontal, or any angle in between.

  • The shear sense across the fault

    • The direction that material on one side of the fault moved relative to the material on the other side.

• Dip-slip faults: faults that move along the direction of the dip plane.

• Strike-slip fault: the slip direction is parallel to a horizontal line on the fault surface.

• Oblique-slip fault: sliding occurs diagonally on the fault surface. Combination of a dip-slip and strike-slip fault.

Recognizing Faults

• Fault scarp: a small step on the ground surface where one side of a fault has moved vertically with respect to the other.

• Slickensides: polished fault surfaces.

9.4 Folds and Foliation

Geometry of Folds

• Fold: a curve in the shape of a rock layer.

• Parts of a fold:

  • Limbs: the sides of the fold that have less curvature.

  • Hinge: the line along which the curvature of the fold is greatest.

  • Axial surface: an imaginary plane that contains the hinge lines of successive layers and effectively divides the fold into two halves.

• Using these parts, we can distinguish:

  • Anticlines, synclines, and monoclines

  • Nonplunging and plunging folds

  • Domes and basins

Formation of Folds

• Folds develop in two ways

  • Flexural-slip folds: process where a stack of layers bends and slip occurs between the layers to accommodate the bending without producing gaps between layers.

  • Passive-flow folds: form when the rock behaves like weak plastic and slowly flows.

Foliation in Rocks

• Foliation: layering developed when inequant grains align in response to deformation.

9.5 Causes of Mountain Building

• Mountains form primarily in response to convergent boundary deformation, continental collisions, and rifting

Mountains Related to Subduction

• Compressional stress develops and drives crustal shortening in the overriding plate, producing a fold-thrust belt.

Mountains Related to Collision

• During collision, intense compression generates fold-thrust belts on the margins of the orogen.

  • Dynamothermal metamorphism occurs in the interior of the orogen.

  • Passive-flow folds and tectonic foliation also form at this time.

  • Suture: the boundary between blocks that had been separate before collision.

Mountains Related to Continental Rifting

• Continental rift: area where a continent undergoes stretching.

• Tensional stress causes normal faulting in upper crust.

• Movement on normal faults drops down blocks of crust, typically tilt over as they move.

• Rifts contain several elongated mountain ranges (the tilted blocks of crust) separated by deep, sediment-filled basins.

• Stretching during rifting thins the lithosphere, resulting in decompression melting.

• Magma is then produced, which may cause uplift of the rift and its borders.

Measuring Mountain Building

• The global positioning system provides a means to measure rates of uplift and horizontal shortening in orogens.

9.6 Other Consequences of Mountain Building

• Produces new rocks

• Results in crustal uplift

• Creates distinctive landforms

Forming Rocks In and Near Mountains

• Igneous activity during orogeny - melting takes place at convergent boundaries in the mantle above the subducting plate.

• Sedimentation during orogeny - weathering and erosion occurs in mountain belts.

• Metamorphism during orogeny - occurs where mountain building thrusts one part of the crust over another, resulting in rock being subjected to high temperature and pressure.

Processes That Cause Uplift and Produce Mountainous Topography

• Isostasy or isostatic equilibrium: The condition that exists when the buoyancy force pushing lithosphere up equals the gravitational force pulling lithosphere down.

• Geological processes that can change the thickness or density of layers in the lithosphere.

  • Crustal shortening and thickening

  • Adding igneous rock to the crust

  • Removal of lithospheric mantle

  • Thinning and heating of lithosphere

What Goes Up Must Come Down

• As slopes rise, erosion and landslides bring rock down.

• Uplift and erosion happen simultaneously.

• For the elevation of a range to increase over time, the rate of uplift must exceed the rate of erosion.

• If uplift rate becomes less than erosion rate, the elevation of the range decreases.

9.7 Basins and Domes in Cratons

• Craton: consists of crust that has not been affected by orogeny for at least a billion years.

• Cratons have cooled substantially and become strong and stable.

• Cratons are divided into two provinces: shields and cratonic platforms

  • Shields: an older, interior region of a continent

  • Cratonic platforms: where a relatively thin layer of Phanerozoic sediment covers the Precambrian rocks.