deformation 2
Topic: Deformation: Folding and Faulting (Continued)
Outline
How Rocks Strain (Deform)
Basic Deformation Structures
Reading
Text, Chapter 7
How Rocks Strain (Deform)
Introduction to Rock Deformation
The way rocks strain or deform is influenced by various factors including the nature of forces (stresses) acting on them, their physical properties (e.g., mineral composition, strength), temperature, confining pressure, and the rate at which stress is applied (strain rate).
Rocks respond to stress by undergoing strain, which can manifest as elastic, ductile, or brittle deformation.
Major Stresses in the Crust
In the upper crust, the primary stresses affecting rocks are tectonic stresses, which are predominant at plate boundaries. These stresses arise from the movement of Earth's lithospheric plates.
Types of Tectonic Stresses:
Tensional Stresses
Described as pull-apart stresses, where forces pull a rock body apart, leading to extension and thinning of the crust. These dominate at divergent plate boundaries, such as mid-ocean ridges and rift valleys (e.g., the East African Rift).
Compressional Stresses
Also known as squeezing stresses, where forces push a rock body together, resulting in shortening and thickening of the crust. These prevail at convergent plate boundaries, such as subduction zones (e.g., the Andes Mountains) and continental collision zones (e.g., the Himalayas).
Shearing Stresses
These push parts of a rock body in opposite directions across one another, resulting in parallel displacement or distortion without significant volume change. They are prominent at transform plate boundaries, exemplified by large strike-slip faults like the San Andreas Fault.
Geological Features
The lecture discusses the positions of multiple tectonic plates, illustrating how their interactions generate the aforementioned stresses and result in deformation:
Eurasian plate
North American plate
Australian plate
Filipino plate
Juan de Fuca plate
Cocos plate
Caribbean plate
Arabian plate
Indian plate
Pacific plate
Nazca plate
African plate
South American plate
Antarctic plate
Scotia plate
Deformation Under Different Conditions
Brittle Deformation
Occurs under relatively low confining pressures and temperatures, typically found in the upper few kilometers of the crust where rocks are rigid. Higher strain rates also tend to promote brittle failure.
Characterized by fracturing (faulting), where materials undergo little elastic or ductile deformation under increasing stress before breaking suddenly and permanently. This rupture occurs when the stress exceeds the rock's strength.
Example Terms:
Hanging Wall - The block of rock that lies structurally above an inclined fault plane. It is called the 'hanging wall' because a miner could theoretically hang a lantern from it.
Foot Wall - The block of rock that lies structurally below an inclined fault plane. A miner would stand on this block.
Fault Types:
Normal Fault - Caused by tensional stress where the hanging wall moves down relative to the footwall.
Strike-slip or Transverse Fault - Caused by shearing stress where blocks move horizontally past each other.
Reverse or Thrust Fault - Caused by compressional stress where the hanging wall moves up relative to the footwall.
Ductile Deformation
Happens under higher confining pressures and temperatures found deeper in the crust (typically several kilometers below the surface), where rocks become more plastic due to increased atomic mobility and confining pressure preventing fracture. Slower strain rates also favor ductile behavior.
Rocks deform by bending (folding) rather than fracturing, indicating smooth, continuous plastic deformation without loss of cohesion. The material changes shape permanently but does not break.
Example Terms:
Anticline - An arch-like fold where the rock layers (beds) dip away from the central hinge line. The oldest rock layers are typically found in the core of an anticline.
Syncline - A trough-like fold where the rock layers (beds) dip toward the central hinge line. The youngest rock layers are typically found in the core of a syncline.
Geological Structures Associated with Folding
Types of Folds:
Anticlines
Appearance and characteristics: Looks like an arch, with limbs dipping away from the axial plane. Oldest strata are exposed in the core.
Synclines
Appearance and characteristics: Looks like a trough, with limbs dipping toward the axial plane. Youngest strata are exposed in the core.
Monoclines
Appears as a stair step-like structure, consisting of a localized bend or flexure in otherwise horizontal or uniformly dipping layers. They are often draped over a fault block in the underlying basement rock, which did not propagate through the overlying sedimentary layers.
Plunging Anticlines
Characterized by a tilted hinge line (or fold axis) that is not horizontal. In outcrop, plunging anticlines typically form V-shaped or U-shaped patterns that point in the direction of the plunge.
Plunging Synclines
Also characterized by a tilted hinge line. In outcrop, plunging synclines typically form V-shaped or U-shaped patterns that point opposite to the direction of the plunge.
Dome
A large, upwardly bowed, roughly circular or elliptical structure where rock layers dip outward in all directions from a central point. Structurally, it resembles an overturned bowl, with the oldest rocks in the center.
Basin
A large, downwardly bowed, roughly circular or elliptical structure where rock layers dip inward in all directions toward a central point. It appears as an upright bowl, with the youngest rocks in the center.
Summary of Key Definitions
Brittle Material:
A substance that displays minimal elastic or plastic deformation until breaking rapidly and permanently under increased stress (faulting). Its constituent bonds break rather than flow, leading to a sudden failure.
Ductile Material:
A substance that can undergo smooth and continuous plastic deformation (folding) under increasing force, changing shape permanently without fracturing. When the force is released, it does not return to its original shape, indicating permanent flow.
Basic Deformation Structures
The two primary types of deformation structures mentioned are:
Folds
Represent ductile deformation, characterized by flexures or bends in rock layers. They form under compressional stress at greater depths and higher temperatures.
Faults
Represent brittle deformation, characterized by fractures in rocks, along which displacement has occurred. They form when stresses exceed the rock's strength, typically under lower temperatures and confining pressures.
Types of Faults
Normal Faults
Occur when the hanging wall moves down relative to the footwall. These are characteristic of tensional stress and crustal extension, leading to the lengthening and thinning of the crust, often forming grabens and half-grabens.
Reverse Faults
Occur when the hanging wall moves up relative to the footwall. These are characteristic of compressional stress and crustal shortening, leading to the thickening of the crust. If the fault plane has a dip angle of or less, it is specifically called a thrust fault. Reverse faults typically have steeper dip angles (greater than ).
Strike-slip Faults
Characterized by horizontal movement along the fault line; there is no significant vertical displacement of the hanging wall or footwall. They are caused by shearing stresses.
Left-lateral strike-slip fault: If you stand on one side of the fault and look across, the block on the other side has moved to your left.
Right-lateral strike-slip fault: If you stand on one side of the fault and look across, the block on the other side has moved to your right.
Connection Between Fault Motions and Earthquakes
It is the sudden, episodic movements along these faults, as accumulated strain energy is rapidly released, that are directly responsible for causing earthquakes. When rocks on either side of a fault slip past each other, they generate seismic waves that travel through the Earth.