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Normal Faults - Structural Geology - Lecture Notes, Study notes of Geology

West Bengal State UniversityGeology

In these Lecture notes, Professor has tried to illustrate the following points : Normal Faults, Hanging Wall, Footwall, Younger Rocks, Crustal Extension, Driving Process, Older Rocks, Separation, Relative Stratigraphic, Fault Plane

Typology: Study notes

2012/2013

Uploaded on 07/22/2013

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I. Normal Faults
A. Characteristics
1. Defined
a. Inclined dip-slip faults in which hanging wall has moved down
relative to footwall
(1) younger rocks placed on older rocks by fault
(2) Crustal extension = driving process
2. Separation and Normal Faulting
a. Relative stratigraphic offset of normal fault a function of:
(1) strike and dip of fault plane
(2) strike and dip of bedding
(a) controls outcrop and cross-section rock patterns
3. Folds Associated with Normal Faults
a. Drag Folds
b. Listric normal faults and roll-over anticlines
(1) listric fault: angle of dip decreases with increasing depth
(a) concave upward normal fault
(2) Roll-over Anticline
(a) As hanging wall block downdrops, beds deform to
maintain contact with footwall
4. Associated Deformation
a. drag folds
b. slickensides
c. cataclastites
d. ducile shear at deeper levels
B. Shape and Displacement of Normal Faults
1. Surface Trace
a. Sinuous vs. straight fault trace
b. Mountain front geomorphology
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I. Normal Faults A. Characteristics

  1. Defined a. Inclined dip-slip faults in which hanging wall has moved downrelative to footwall

(1) younger rocks placed on older rocks by fault (2) Crustal extension = driving process

  1. Separation and Normal Faulting a. Relative stratigraphic offset of normal fault a function of: (1) strike and dip of fault plane (2) strike and dip of bedding (a) controls outcrop and cross-section rock patterns
  2. Folds Associated with Normal Faults a.b. Drag FoldsListric normal faults and roll-over anticlines (1) listric fault: angle of dip decreases with increasing depth (a) concave upward normal fault (2) Roll-over Anticline (a) As hanging wall block downdrops, beds deform to maintain contact with footwall
  3. Associated Deformationa. drag folds b. slickensides c.d. cataclastitesducile shear at deeper levels B. Shape and Displacement of Normal Faults
  4. Surface Trace a.b. Sinuous vs. straight fault traceMountain front geomorphology
  1. Shape at Depth a. listric normal b. high-angle normal faults: constant dip at depths c. Detachment faults: major fault surface of "rock detachment"(1) commonly low angle d. Imbricate Faults: parallel fault sets e. rotational vs. non-rotational normal faults C. Structural Associations
  2. small scale vs. large scale faulting a. outcrop vs. mountain front size faults
  3. synthetic vs. antithetic faults a. synthetic: smaller scale faults parallel to major faults b. antithetic: smaller scale faults oriented with dip in opposite direction of major faults
  4. Horsts and Grabens (fault bounded blocks) a. Horsts: uptrown fault-bounded blocks b. Grabens: down-thrown fault-bounded blocks (1) Half-grabens: normal faulting on one side of block, with net downward rotation c. Basin and Range of Western U.S. (1) Grabens as tectonic sediment traps
  5. Regional Fault Systems a. Rift-Extension Systems (1)(2) Basin and range of Western U.S.Triassic Rift Basins of Eastern U.S. b. Gulf Coast (1)(2) Extensive sedimentation by sediment delivered to coastGrowth Faults (a) Normal faulting that develops contemporaneously with sedimentation
  1. Stratigraphic a. place older rocks on top of younger rocks(1) repetition of stratigraphic sequences (2) mismatched facies sequences (a) e.g. marine over fluvial
  2. Thrus-fold relations C. Shape and Displacement of Thrust Faults
  3. Shape of Thrusts a. Common patterns:(1) thrusts listric at depth (2) occur in branching networks that connect back to single (3) main fault at depththrust faults have irregular surface geometry with up and down ramps common(a) front ramps (b) side ramps b. Klippe: isolated erosional remnant from allochthon c. Fenster (window): erosional hole through hanging wall thrust block, into footwall thrust block D. Structural Environments and Thrusting
  4. Local Thrusts a. Diapiric Structures (1) less dense rock bodies that are uplifted by buoyantisostatic forces through more dense rock

(a)(b) e.g. Salt DomesDiapiric igneous intrusives (2) Up-thrust systems common in strata above diapiric intrusion b. Thrust Systems (1) Foreland Fold and Thrust Belts (a) major orogenic belts characterized by compression,thrusting and folding (2) Terms and concepts

(a) salient: convex wedge of thrust toward foreland (b) Reentrant: concave bend in thrust toward foreland (c) Decollement: surface of detachment, above which= thrust sytem, below which = undeformed basement rocks (d) Thrust Duplex i) imbricate thrust faults that form horsesbetween them ii) horse = blocks of rock bounded by thrust faults (e) Tear Faults i) vertical high-angle faults, that form perpendicular to thrust trace ii) accommodation of deformation of rocksheet

III. Strike Slip Faults A. Definitions and Terminology

  1. Strike Slip Faults a. offset parallel to strike of fault plane (1) commonly high angle faults (2) dextral vs. sinistral strike slip faults b. Tear faults: secondary strike-slip faults associated with thrust systems c. Wrench fault: another term for high-angle strike-slip fault d. Transform or transcurrent faults: major strike slip faults, some of which form tectonic plate boundaries (1) hundreds of km of displacement, e.g. San Andreas B. Characteristics of Strike-Slip Faults
  2. horizontal strike separation of strata along fault plane a.b. mismatched stratigraphyexotic terranes
  1. Terminations a. branching and bending terminations (1)(2) thrust terminationsnormal termination

D. Structural Associations

  1. Transform Tectonics a. e.g. San Andreas system