CHAPTER 7
Deformation and Metamorphism
Why Study Folds, Faults, and Other Records of Rock Deformation?
• Deformation creates our landscape.
• Folds: Petroleum accumulation.
• Faults: Study earthquakes hazards.
• Intellectual interest: Why are fossiliferous limestones found at the top of Mt. Everest?
Surface Processes versus Internal Processes
• Surface processes (sculpture the physical landscape) - weathering, mass wasting, and erosion.
• Internal processes (tectonic forces) - folding/faulting, earthquakes, mountain building and plate tectonics.
Deformation
• Changes in volume or shape of a rock body.
• Confining pressure
o Pressure that is uniform in all directions.
o Results in a reduction in volume.
Not a large effect if solid or liquid.
o Affects the way rocks behave when subjected to directional forces.
Low confining pressure = brittle behavior (fracture)
High confining pressure = ductile behavior (flow)
• Directional stresses
o Not uniform in all directions.
o Results in changes in shape and/or volume.
o Compressional stresses.
Act to shorten a rock body.
Change the shape and the volume.
Produce both faults (fractures) and folds.
Reverse faults.
Folds.
Hybrid.
Blind-thrust fault.
The stress at convergent plate boundaries.
o Tensional stresses
Act to lengthen or extend a rock body.
Change the shape and the volume.
Produce faults.
Normal faults.
The stress at divergent plate boundaries.
o Shear stresses
Act to shear or cause slip in a rock body
Change the shape
Produce faults
Right-lateral strike-slip.
Left-lateral strike-slip.
The stress at transform plate boundaries.
• Types of deformation
o Elastic deformation
Results from small to moderate forces
Reversible (below the elastic limit)
o Plastic deformation
Results from moderate to large forces.
Irreversible - results in permanent deformation.
o Influenced by the mineral composition
Quartzite, granite, and gneiss are strong and brittle
Salt, gypsum, marble, and shale are more ductile
o Influenced by geologic time.
o Influenced by confining pressure and temperature.
Low temperature = brittle behavior (fracturing)
High temperature = ductile behavior (folding and flowing)
• Rock structures are the results of deformation.
Strike and Dip
• Allow geologists to reconstruct the orientation and shape of geologic structures.
• Strike - the trend or direction of a layer or surface relative to north;
the direction of a horizontal line in the layer.
• Dip - the angle of inclination of the surface relative to the horizontal;
the angle of maximum inclination measured perpendicular to the strike.
• Symbols are plotted on a map.
Folds - wave-like undulations in rock caused by compression.
• Characteristics and terminology of folds.
o Limbs - sides of a fold.
o Axis - line drawn along the points of maximum curvature.
o Plunge - the inclination of the axis relative to the horizontal.
o Axial plane - an imaginary surface that divides the fold as symmetrically as possible.
• Types of folds.
o Anticlines
Upfolds or arches
Strata get older toward the core of the fold.
o Synclines
Downfolds or troughs
Strata get younger toward the core of the fold.
o Monoclines
Flexures in strata, or folds with only one limb.
May overlie vertical faults in the underlying rocks.
Common on the Colorado Plateau.
o Plunging folds.
o Domes.
Doubley plunging anticlines
Compare with basins.
o Symmetrical folds
Limbs on either side diverge at the same angle.
Compare with asymmetrical folds.
o Overturned fold.
One limb is tilted beyond the vertical.
o Recumbent fold
Axial plane of fold is nearly horizontal.
Faults
• Fractures in the crust along which appreciable displacement has taken place.
• Concept of relative displacement.
• Sudden movements along faults cause earthquakes.
• The grinding of one side of the fault past the other side produces:
o Fault gouge - pulverized rock
o Slickensides - washboard-like groves or striations on the fault surfaces parallel to the displacement direction.
• Fault trace - the line of intersection of the fault with the earth's surface.
Types of Faults
• The type you get reflects the stress regime - the directions and magnitudes of the three principal stresses.
• Dip-slip faults - movement is primarily vertical.
o Hanging wall versus foot wall
o Normal faults.
Hanging wall moves down relative to the footwall
Products of tensional stresses or extension - the maximum principal stress is vertical; the minimum principal stress is horizontal and in the direction of extension. Fault plane generally dips at an angle of about 60°
Often responsible for horst and graben structures and fault-block mountains.
When active they form prominent fault scarps - topographic steps.
Prevalent in the Great Basin or Basin and Range Province of western U.S.
Termed listric if they shallow with depth.
o Reverse faults.
Hanging wall moves up relative to the footwall.
Products of compressional stresses - the minimum principal stress is vertical; the maximum principal stress is horizontal and in the direction of compression. Fault plane generally dips at an angle of about 45° to 60°.
Thrusts are low-angle reverse faults (dip less than 45°)..
Common in zones of continental collision.
• Detachments or surfaces of decollement
o Nearly horizontal faults
o May form the interface between the brittle upper crust and the ductile lower crust. The brittle upper crust is "detached" or decoupled from the ductile lower crust.
o Thrusts and listric normal faults may "root" or "sole" into detachments.
• Strike-slip faults - movement is dominantly horizontal.
o Products of horizontal compression and/or shear - the maximum and minimum principal stresses are both horizontal and at an angle of about 45° to the fault plane.
o Right-lateral - opposite side of fault moves to the right of an observer looking across the fault.
o Left-lateral - opposite side moves to the left.
o Transform - a plate-boundary strike-slip fault usually connecting spreading-ridge segments. The ridge segments appear to be offset in a sense opposite to the direction of slip. The San Andreas fault is an example.
o Topographic features include:
Offset drainages, sag ponds, linear valleys, scarps and shutter ridges
• Oblique-slip faults - movement is a combination of dip-slip and strike-slip.
Joints
• Fractures with no appreciable displacement that come in sets, often with regular spacing. Products of:
o Cooling
o Unloading
o Stresses associated with regional crustal deformation
• Can influence regional drainage patterns and erosion.
• Form underground conduits for the movement of groundwater and hydrothermal solutions.
Metamorphic Rocks
• Products of elevated temperature, pressure, and chemically active fluids.
• An important component of mountain belts, both young and old.
• Often highly deformed and intruded by igneous rocks.
• Provide important clues about geologic processes operating within the earth's crust.
Metamorphism
• The process of changing the mineralogy and texture of a pre-existing rock by subjecting it to elevated temperature (T), pressure (P), and chemically active fluids.
o High grade metamorphism involves extreme P&T, near the melting point.
o Low grade metamorphism involves P&T conditions only slightly higher than those associated with the lithification of sediments.
o Hot circulating fluids not only affect the metamorphic process itself, but can greatly extend the zone of metamorphism by the transfer of heat and chemically reactive ions.
Metamorphic Environments
• Three basic types of metamorphism:
o Regional metamorphism - Results from intense differential stresses and high temperatures associated with large-scale deformation during mountain building.
o Contact metamorphism - Results from intense heat when a rock is near or in contact with an igneous intrusion.
o Cataclastic metamorphism - Results from shearing or grinding stresses along fault zones.
• In all cases fluids are usually involved.
• The deformation accompanying metamorphism is often remarkably similar at all size scales.
Agents of Metamorphism
• Heat
o Geothermal gradient is ~20-30 degrees C per km.
o Provides the energy to drive chemical reactions.
o Expels water from hydrous minerals such as clay.
o May be very intense along contacts with molten magma.
• Pressure/stress
o Chemical reactions are pressure-dependent.
o Confining pressure - forces act equally in all directions.
o Differential stress - forces are unequal in different directions and tend to
shorten (compression), stretch (tension), or shear the rock.
o Shear stresses impart texture or fabric to a metamorphic rock and cause preferred orientations of mineral grains.
• Chemically active fluids
o Some water in the pore spaces of virtually every rock.
o Many minerals contain water in their crystalline structure which can be released at elevated P&T.
o Pore waters contain additional ions available for chemical reactions.
o Water acts as a catalyst by aiding ion migration.
o Free waters accompanying metamorphic processes are called hydrothermal solutions.
Textural and Mineralogical Changes
• The degree of metamorphism is reflected in the texture and mineralogy of metamorphic rocks.
o Rocks subjected to low grade metamorphism become more compact and dense.
o Under more extreme conditions, recrystallization occurs.
o Mineral alignment occurs reflecting directional stresses, giving rocks a banded appearance termed foliation.
o Foliation forms perpendicular to directed stress.
o Types of foliation (increasing P&T):
Slaty cleavage: splits into slabs along tiny aligned mica grains.
Schistosity: scaly texture due to alignment of large mica grains.