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Offset Features - Field Geology - lecture notes, Study notes of Geology

University of AllahabadGeology

Professor has put stress on the following points in these Lecture Notes Offset Features, Originated, Joints, Dikes, Recognition, Rock Bodies, topographic Features, Breccia, Fault Gouge, Slickensides

Typology: Study notes

2012/2013

Uploaded on 07/18/2013

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8/19/05 1
FIELD EXAMPLES OF FAULTS
IMain Topics
AField examples of faults
BField examples of faults that originated from joints and dikes
II Field examples of faults
ARecognition of faulting
1Offset features (rock bodies & topographic features)
2Juxtaposition of markedly different rock bodies
3Fault gouge or breccia
4Slickensides and slickenlines
BRecognition of recent or active faulting
1Displaced features that are geologically young
aFault scarps: vertically displaced ground surface resulting from
dip-slip faulting (normal and reverse faults)
Beware of interpretation of
fault-line scarps
bOffset topography (e.g., shutter ridges)
cLaterally offset streams (strike-slip faults)
dOffset cultural features
2Fault topography
aFaceted spurs (normal faults)
bSag ponds (strike-slip faults)
cLinear hillside benches (strike-slip faults)
3Historic seismicity
4Quaternary seismicity
5Style of faulting (stick-slip vs. creep) can vary along faults
aYoung" sedimentary rock ⇒ creep
bCrystalline rock ⇒ stick slip.
CSegmented structure of faults
1Splay cracks or tail cracks (dilatant fractures) form at localized
tensile stress concentrations.
2These cracks can link small faults together.
3Several major geothermal fields occur at linkages between strike-
slip faults.
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8/19/05 1

FIELD EXAMPLES OF FAULTS

I Main Topics A Field examples of faults B Field examples of faults that originated from joints and dikes II Field examples of faults A Recognition of faulting 1 Offset features (rock bodies & topographic features) 2 Juxtaposition of markedly different rock bodies 3 Fault gouge or breccia 4 Slickensides and slickenlines B Recognition of recent or active faulting 1 Displaced features that are geologically young a Fault scarps: vertically displaced ground surface resulting from dip-slip faulting (normal and reverse faults) Beware of interpretation of fault-line scarps b Offset topography (e.g., shutter ridges) c Laterally offset streams (strike-slip faults) d Offset cultural features 2 Fault topography a Faceted spurs (normal faults) b Sag ponds (strike-slip faults) c Linear hillside benches (strike-slip faults) 3 Historic seismicity 4 Quaternary seismicity 5 Style of faulting (stick-slip vs. creep) can vary along faults a Young" sedimentary rock ⇒ creep b Crystalline rock ⇒ stick slip. C Segmented structure of faults 1 Splay cracks or tail cracks (dilatant fractures) form at localized tensile stress concentrations. 2 These cracks can link small faults together. 3 Several major geothermal fields occur at linkages between strike- slip faults.

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8/19/05 2

4 Several major oil basins occur at dilatant steps along strike-slip faults ("mega-sag ponds") D Geomechanical effects of faulting 1 Splay cracks near ends of fault traces or ends of fault segment traces (mode II effect) 2 Buckles (mode II effect) 3 Echelon fractures (mode III effect)

Splay Buckle crack

Mode II Effects

Mode III Effects

Echelon crack array

4 The mean normal stress becomes more compressive at right steps between right-lateral faults (dilatant steps) and less compressive at right steps between left-lateral faults ("anti-dilatant steps") and vice-versa.

III Field examples of faults that originated from joints and dikes References Segall, P. and Pollard, D.D., 1980, Mechanics of discontinuous faults: Journal of Geophyscial Research, v. 85, p. 4337-4350. Brown, R. D., Jr., and Kockelman, W.J., 1983, Geologic principles for prudent land use: a decisionmaker's guide for the San Francisco Bay region: U.S. Geological Survey Professional Paper 946, 97 p.

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