Rock Structure and Fault Activity chapter 9. What is structural geology The study of the forms of the Earth’s crust and the processes which have shaped

Rock Structure and Fault Activity

chapter 9

What is structural geology

The study of the forms of the Earth’s crust and the processes which have shaped it

• analysis of displacement and changes in shape of rock bodies (strain)

• reconstruct stress that produced strain

Structural DeformationRocks deform when

stresses placed upon them exceed the rock strength

• Brittle deformation (e.g. fractures)

• ductile deformation (e.g. folds)

Driving Forces• Plate tectonics – plate convergence and ridge

spreading• Deep burial of sediments• Forceful intrusion of magma into the crust• Meteorite impacts

Evidence of Crustal Deformation

• Folding of strata• Faulting of strata• Tilting of strata• Joints and fractures







Applications of structural geology

• subsurface exploration for oil and gas

• mining exploration• geotechnical investigations• groundwater and environmental

site assessment

Geological structures

• Geologic bed contacts• Primary sedimentary structures• Primary igneous structures• Secondary structures

Fundamental Structures

Three fundamental types of geologic structures:

• bed contacts• primary structures - produced during

depositionor emplacement of rock body• secondary structures - produced by

deformationand other process after rock is emplaced

Bed Contacts

Boundaries which separate one rock unit from another

• two types:1. Normal conformable contacts2. Unconformable contacts

(‘unconformities’)

Conformable Bed Contacts

Horizontal contact between rock units with no break in deposition or erosional gaps

• no significant gaps in geologic time

Book Cliffs, central Utah

Unconformable Contacts

Erosion surfaces representing a significant break in deposition (and geologic time)

• angular unconformity• disconformity• non-conformity

Angular UnconformityBedding contact which discordantly cuts

across older strata• discordance means strata are at an angle to

each other• commonly contact is erosion surface

Formation of an angular unconformity

DisconformityErosional gap between rock units

without angular discordance• example: fluvial channel cutting

into underlying sequence of horizontally bedded deposits

Nonconformity

Sedimentary strata overlying igneous or metamorphic rocks across a sharp contact

• example: Precambrian-Paleozoic contact in Ontario represents a erosional hiatus of about 500 ma

Grand Canyon, USA

Structural Relations

The structural relations between bed contacts are important in determining:1. presence of tectonic

deformation/uplift and;2. relative ages of rock units

• principle of original horizontality• principle of cross-cutting• principle of inclusion

Principle of Original Horizontality

Sedimentary rocks are deposited as essentially horizontal layers• exception is cross-bedding (e.g. delta foresets)• dipping sedimentary strata implies tectonic uplift and

tilting or folding of strata

Principle of Cross-cutting

Igneous intrusions and faults are younger than the rocks that they cross-cut

Mafic dike cutting across older sandstones

Cross-cutting Relations

Often several cross-cutting relationships are present

• how many events in this outcrop?

Principle of Inclusion

Fragments of a rock included within a host rock are always older than the host

Fundamental Structures

Three fundamental types of structures:

• bed contacts• primary structures• secondary structures

Primary Sedimentary Structures

Structures acquired during deposition of sedimentary rock unit

Stratification - horizontal bedding is most common structure in sedimentary rocks

http://wrgis.wr.usgs.gov/docs/parks/rxmin/laminSS215x192.jpg


Cross-bedding - inclined stratification recording migration of sand ripples or dunes


Ripples - undulating bedforms produced by

unidirectional or oscillating (wave) currents

Ripple marks


Graded bedding - progressive decrease in grain size upward in bed• indicator of upwards direction in deposit• common feature of turbidites


Mud cracks - cracks produced by dessication of clays/silts during subaerial exposure


Sole marks - erosional grooves and marks formed by scouring of bed by unidirectional flows

• good indicators of current flow direction


Fossils – preserved remains of organisms, casts or moulds

• good strain indicators• determine strain from change in shape of fossil• relative change in length of lines/angle

between lines

Primary Igneous Structures

Flow stratification • layering in volcanic rocks produced

by emplacement of successive lava sheets

• stratification of ash (tephra) layers


Flow stratification • layering in volcanic rocks produced

by emplacement of successive lava sheets

• stratification of ash (tephra) layers


Pillow lavas - record extrusion and quenching of lava on sea floor

Importance of Primary Structures

1. Paleocurrents - determine paleoflow directions

2. Origin – mode of deposition, environments3. Way-up - useful indicators of the direction of

younger beds in stratigraphic sequence4. Dating - allow relative ages of rocks to bedetermined based on position, cross-cuttingrelations and inclusions5. Strain indicators - deformation of primarystructures allows estimates of rock strain

Secondary Structures

Secondary structures - deformation structures

produced by tectonic forces and other stresses in crust

Principle types:• fractures/joints• faults/shear zones• folds• cleavage/foliation/lineation

Fractures and Joints

Fractures – surfaces along which rocks have broken and lost cohesion

Joints - fractures with little or no displacement parallel to failure surface

• indicate brittle deformation of rock

Fractures and Joints

FaultsFaults - fracture surfaces with appreciable displacement of strata

• single fault plane

• fault zone - set of associated shear fractures

• shear zone - zone of ductile shearing

Shear Zones

Shear zone - zone of deformed rocks that are more highly strained than surrounding rocks

• common in mid- to lower levels of crust

• shear deformation can be brittle or ductile

Fault Terminology

Hanging wall block- fault block toward which the fault dips

Footwall block - fault block on underside of fault

Fault plane – fault surface

Fault Slip

Slip is the fault displacement described by:• direction of slip• sense of slip• magnitude of slip

Fault Types

Dip-slip faults - slip is parallel to the fault dip directionnormalreversethrust

Fault Types

Normal fault - footwall block dispaced up

Fault Types

Reverse (thrust) fault - footwall block displaced down

Fault Types

Strike-slip – fault slip is horizontal, parallel with strike of the fault plane• right-handed (dextral)• left-handed (sinistral)

Fault Types

Oblique slip – Combination of dip- and strike-slip motion• dextral-normal• dextral-reverse• sinistral-normal• sinistral-reverse

Faults

What type of faults are shown here?

Faults


Faults


Faults


Folds

Folds – warping of strata produced by compressive deformation• range in scale from microscopic

features to regional-scale domes and basins

• indicators of compression and shortening

Fold Terminology

Hinge (Axial) plane - imaginary plane bisecting fold limbs

Hinge line - trace of axial plane on fold crestPlunge - angle of dip of hinge line

horizontal fold axis

plunging fold axis

Fold Terminology

Anticline - convex in direction of youngest beds

Syncline - convex in direction of oldest beds

Antiform - convex upward fold (stratigraphy unknown)

Synform - concave upward fold

Anticline / Antiform?

SynclineSynform

?

Fold Terminology

Synformal Anticline - overturned anticline

Antiformal Syncline - overturned syncline

Fold Terminology

Monocline - step-like bend in strata

Foliation and CleavageFoliation - parallel alignment of planar fabric elements

within a rockCleavage - tendency of rock to break along planar surface

cleavage is a type of foliation• resemble fractures but are not physical discontinuities

Foliation and CleavageFoliation - parallel alignment of planar fabric elements

within a rockCleavage - tendency of rock to break along planar surface

cleavage is a type of foliation• resemble fractures but are not physical discontinuities

Lineations

Lineation - sub-parallel to parallel alignment of elongate linear fabric elements in a rock body

• e.g. slickenlines and grooves on fault plane surface

Structural analysis

Involves three steps1. Descriptive or geometric analysis2. Kinematic analysis3. Dynamic analysis

Geometrical analysis

Measurement of the 3-dimentional orientation and geometry of geological structures

simplified into:

• lines• planes

lines or linear geological structures

• liniation – any linear feature observed in a rock or

on a rock surface – any imaginary line – such as a fold axis

Orientation of linear structures

LINESTrend – azimuth direction measured clockwise from north 360°Plunge – angle of inclination of line measured from the horizontal (0 - 90°)

Examples of linear structures

• Primary – flute casts, grooves, glacial striae• Secondary – slickenlines, mineral lineations

Glacial striations on bedrock sole marks

http://almandine.geol.wsu.edu/~geo101/sckuehn/Sect3-Sp99/weathering/glacial-striae-chernicoff-cropped.jpg

Examples of linear structures

• Primary – flute casts, grooves, glacial striae• Secondary – slickenlines, mineral lineations

Grooves on fault plane Slickenlines on fault surface

Orientation of Linear Structures

linear structures on an other planar surface:

• pitch angle– angle from horizontal measured

within the planePitch angleStriations

on a fault plane

Planar Geological Structures

• bedding planes and contacts• foliation• joint surfaces• fault planes• fold limbs• fold axial planes (imaginary

surface)

Examples of Planar Structures

Bedding planes – most common • primary depositional surface• erosional surface

inclined bedding plane


Foliation – cleavage planes produced by metamorphism

• common in slates and phyllites

foliated phyllite


Joint planes – planar fracture surfaces caused by brittle failure


Fold axial plane - imaginary plane bisecting limbs of fold

Orientation of Planar Structures

The attitude of a plane can be established from any two lines contained in the plane, provided they are not parallel


Strike – azimuth direction of a horizontal line in a plane

Dip – angle of inclination of line measured from the horizontal (0 - 90°)


Appearent dip– dip measured

along a line other than 90 to strike

– apparent dip will always be less than the true dip angle

Measurement of orientation

Strike (plane) Trend (line)azimuth orientation measured with a compass

Measurement of orientation

Strike (plane) Trend (line)azimuth orientation

measured with a compass

Dip (plane)Plunge (line)inclination measured

using an inclinometer

Measurement of Strike Direction

Right hand rule???When your thumb (on your right

hand) is pointing in the direction of strike your fingers are pointing in the direction of dip!!

Measure of Dip Angle

The angle between the horizontal and the line or plane

Structural Data

Symbols represent different structural data

Symbols are placed on the map:– in the exact field orientation– where the data is measured

Standard Structural Symboles

Exercises

• geological maps• structure contour and structure

maps• three-point problems• cross sections• sterionets

Geological Maps

• distribution of rock types and contacts– symbols on map represent structures

(strike and dip, fold axes, faults etc.)– map and structure symbols allow you

to infer subsurface structures

Outcrop patterns

Outcrop patterns controlled by attitude (strike and dip) of beds and topographic relief

“V” Rule

• Beds dipping downstream “V” –downstream

• Beds dipping upstream “V” – upstream

Vertical beds cut straight

Vertical oriented beds cut in a straight line regardless of topography!!

Horizontal beds

• layers always at the same altitude – do NOT dip in any direction– layered cake

Outcrop Patterns

Which direction are the beds dipping?

Outcrop Patterns


Outcrop Patterns


Outcrop Patterns


Block models

Relations between outcrop patterns and subsurface structures

map view on bottom – cross sections in blocks on top

Bryce 3-D modeling blocks

Structural Contour Maps

Map showing the relief of a subsurface geological surface– top or bottom of bedding planes,

faults or folded surface– constructed from borehole data

Structure Contour Maps

Structure contour lines are lines of equal elevation• show elevation relative to horizontal datum• values are often negative since subsurface

elevations are commonly below sea level

Datum Surface

Datum is a horizontal reference surface

• regional stratigraphic surface

Constructing Structural Contours

Points of equal elevation along a bed contact• intersection of contact with topo contour• draw structure contours through points of

equal elevation

Planar surfaces

Uniformly dipping plane – contours are parallel

folded planar surfaces

Contours have variable spacing

Rules of Contouring

1) contours cannot cross or bi-furcate2) contours cannot end in the middle of

the map, except at a fault or other discontinuity

3) same contour interval must be used across the map and elevations must be labelled

4) elevation is specified relative to datum (e.g. m above sea level)

Determining Dip

Dip direction and angle can be determined from structure conour maps

• measure horizontal separation X and find difference in Z• tan = Z/X, = tan –1 (Z/X)• e.g. = tan –1 (10m/100m), = 6°

Three-point problem

A minimum of three points are required to uniquely define the orientation of a plane

Three-point problem

• Find min and max values

• Draw line between these and divide distance into intervals

• Connect points of equal elevation

• Two points in a plane at the same elevation lie in the line of strike

Three-point problem

• Find min and max values

• Draw line between these and divide distance into intervals

• Connect points of equal elevation

• Two points in a plane at the same elevation lie in the line of strike

Isochore Map

Drill hole logs giving the thicknesses in the drilled (often vertical) direction

Apparent thickness – true thickness = perpendicular to bedding

Isopach Map

Map showing “true” thickness measured perpendicular to bedding

Cross-sections

Cross-section is a 2-D slice through stratigraphy

• construct perpendicular to dip = true dip

• constructed at any other direction = apparent dip

Engineering properties of faulted or folded rock

• shear strength– loose materials– compressive materials– permeable materials

hydrology of fault zones

• water in fault zones common due to fractured rock– fault zone may be either an aquifer or an

aquiclude• crushed to gravel• crushed to clay

hydrology of fault zones

• water in fault zones common due to fractured rock– fault zone may be either an aquifer or an

aquiclude• crushed to gravel• crushed to clay

Problems due to water in fault zones

• leakage of waste water under a landfill• leakage of water under a dam• sudden collapse and inflow of water into

a tunnel• hydrothermal alteration of rocks to clay

minerals along faults – variable physical, mechanical and hydrological properties

• soluble rocks - cavities

Activity of faults?

• Risk for further movement– active fault – has moved in the last 100 000

to 35 000 years– dormant fault – no recorded movement in

recent history

Indicators of fault movement

• fault scarps• stream displacement • sag ponds lineaments• vegetation displacement

Risk potential depends upon:

1. duration of the quake2. intensity of the quake3. recurrence of the quake

Potential trigger’s

stess > stength• water in a reservoir – added weight and

lubrication• storage of fluids in old mines• blasting• surface excavation• ground water mined from aquifers• extraction of oil and gas from aquifers

Case studies

• Auburn Dam – wide slender arch dam on the American River, upstream of Sacramento, California

• Fig. 9.31• pre investigations

– detailed mapping– 8 km trenches– 2 km exploratory tunnels– 30 km borings

Auburn Dam

• geology– metamorphic competent amphybolite– metasediments– included vertical weak zones and lenses of chlorite

schist, talc schist and talcose serpentinites up to 30 m wide, aligned with foliation

– series of sub parallel mineralized reverse faults with strike transverse to the dam axis dipping 40 to 55 degrees into the abutment

– two of the longest faults are tangential to the dam, close or under the dm on the left abutment

– no active faults in the area– the area was supposed to be low seismicity

Auburn Dam

• foundation construction– earthquake occurred 5.7– regional fault study– reassessment

• 32 km trenches• more borings• surface excavations

aim to establish the time relationship of the faults

Auburn Dam

Concluded that the faults wee formed in another tectonic setting than the present (compressional rather that extensional stress field)

A review of the dam – will it withstand vibrations from a 6.5 magnitude quake on a fault < 8 km from the dam??

“Off set” design recommended to withstand 25 mm to 900 mm

NO DAM built due to discussions on safety!

Baldwin Hill reservoir – failed 1963

• 1 principle embankment, 47 m high, and 5 smaller embankments

• excavated hollow in between at the top of a mountain range

Baldwin Hill reservoir – failed 1963

• geology– friable deposits of the Pliocene Pico Formation, massive

beds of clayey, sandy siltstone– Pleistocene Ingewood Formation. interbedded layers of

sand, silt, and clay, with some thin linestone beds; some of the sand and silt beds are unconsolidated and erodable

– Both formations contain calcareous and limonitic concretions

– bedding dips slightly 5 to 7 degrees, striking roughly parallel to the Inglewood fault

– major active fault, Inglewood, passes just 150 m west of the reservoir

– the fault is a right lateral strike slip with a vertical component

– fault acts as a subsurface dam for a major oil field in the hills

Baldwin

• Excavation phase– 7 minor faults wee mapped– mostly normal faults– 3 to 100 mm silty gouge– largest fault had a total displacement

of more than 8 m

Baldwin

• Design– rock foundation lined with

• asphalt and • gravel drain layer• covered with compacted clay• covered with asphalt

Baldwin

• Construction phase 1947-51– fault 1 caused problems– slide initiated revealing that the fault

passed beneath the inlet/outlet tower– the tower was relocated 48 ft

Baldwin

• after completion– liner cracked along the trace of the fault– emptied in 1957– cracks repaired– cracks were also observed in the surrounding

area of the reservoir– the cracks dipped steeply– trend NS parallel to the faults– some exhibited small sinkholes – indicative of

extensional strain– offset dip slip

Baldwin

• nearby oil fields – oil was being extracted– resulted in subsidence due to collapse of the aquifer– subsidence of 2.7 m between 1917 and 1962

Baldwin

• Failure 1971– emptied completely in 4 hours– seepage along the fault had enlarged

to a pipe– then to a tunnel and – then the collapse of the roof– a canyon eroded completely through

the all of the reservoir

Baldwin

• Failure 1971– Why??

• cracks in the floor extended across the entire reservoir along the trace of the fault

• 50 mm displacement • open voids along the fault• movement along the fault had fractured the lining• rupture of the asphalt membrane• water eroded cavities into the foundation rock

Documents

Rock Structure and Fault Activity chapter 9. What is structural geology The study of the forms of the Earth’s crust and the processes which have shaped