Structural Geology

• Sedimentary rocks deposited in flat and horizontal layers

• They are commonly found to be tilted and bent

• How does this happen

• Folding

– Similar to fold in cloth
– Bent layers of rock

• Faulting

– Plane of rock where it is broken and slipped

• Folding and Faulting

– occur at scales of mm to kilometers
– groups of folds and faults create mountain ranges
– ultimate cause is plate tectonics

Field Data

• Basic measurements made on outcrops
• Exposures:

– May be partly covered
– Structure may be larger than single outcrop

• Require interpolation of data from single outcrops

– Orientation of beds main measurement

• Orientation derived by two measurements

– Strike & dip

• Strike

– Direction of intersection of rock layer with horizontal surface
– Image how water would lap up on tilted rocks
– The line made by the intersection is the strike

• Dip

– Amount of tilting of the rocks, i.e. maximum angle made with the horizontal surface
– Dip always 90º to the direction of strike
 

• Example of description

– Strike 360º, dip 30º west

• Data organized into geologic maps and cross sections
• Maps record of geologic data

– Location of outcrops
– Strike and dips

• Cross sections show features of a slice through the map

Rock Deformation

• Tectonic forces of three types

– Compressive: squeeze and shorten
– Tensional: stretch and pull apart
– Shearing forces: push sides in two directions

• Tectonic forces identical to three plate boundaries

– Convergent (compressive)
– Divergent (tensional)
– Transform (shear)

• Controls on deformation

– Relative difference between differential pressure and confining pressure
– Where very large differential pressure, rock fractures (brittle)
– Where little differential pressure, rock will deform plastically

• Brittle material

– Little change in shape as pressure applied
– Sudden change in shape, i.e. it breaks

• Ductile material

– slow continuous deformation

• Material can change between brittle and ductile behavior

Folding

• Large vocabulary to describe folds

– Describes orientation
– Describe magnitude

• Descriptions important because they relate to forces causing folds
• Forces usually compressive (not tension or shear)

• Anticlines – “arches”, upward bends
• Synclines – “troughs”, downward bends
• Limbs – two sides of fold
• Axial plane – imaginary dividing line at axis of fold
• Fold axis – line of intersection of axial plane with folded beds
• Plunging fold – fold with non-horizontal fold axis, i.e. it dips
 

• Folds may be asymmetrical

– One limb dips more than the other

• Extreme asymmetry called overturned

– one limb is upside down
– its oldest beds are on top

• Most folds eventually disappear

– Plunge into the ground
– Leave a distinct pattern of outcrops

• Domes

– Anticlinal structure
– places where rocks dip away from a central point

• Basins

– Synclinal structure
– Places where rocks dip toward a central point

• Fold belts

– Large (map-scale) folds found in elongate groups
– Indicate major compression
– Example is Valley and Ridge of Appalachians

Fractures: Joints and Faults

• Two kinds of fractures
 

– Joints: a crack without appreciable movement
– Fault: a crack with relative movement on either side of the crack

• Joints

– Common in rocks
– Many causes:

• regional tectonic forces
• Removal of overburden rocks
• Cooling and contraction of volcanic rocks

– Important pathways for water to flow

• Faults

– Result from three types of forces: compressive, tension, and shear
– Different fold types depend on orientation of the forces
– Also by relative motion at the fracture

• Relative motion

– Orientation of fracture (fault) defined by strike and dip
– Dip slip faults: motion along dip of fault

• Normal faults: with gravity
• Reverse faults: against gravity

– Strike slip faults: motion along strike of fault
– Oblique slip faults: motion along strike and dip

• Normal faults: tension

– Divergent boundaries

• Reverse faults: compression

– Convergent boundaries
– Thrust fault: when fault plane < 30º

• Strike slip fault: shear stresses

– Transform boundaries
– Right lateral and left lateral faults

Geologic History

• Stratigraphy and structure

– Layers of rocks
– Deformation events

• Final stage is all that is preserved

– All preceding events can be interpreted