Chapter Iv : Discontinuous Deformation: Structures, Interpretations

CHAPTER IV : DISCONTINUOUS DEFORMATION: STRUCTURES, INTERPRETATIONS

IV-1 STRUCTURES Joints, fractures, stylolitic joints, faults are characteristic structures of discontinuous deformations.

IV-1-1 JOINTS Dry fractures with no displacement. In sedimentary rocks these joints are usually perpendicular or parallel to the bedding plane. In volcanic rocks, the contraction of the rock during cooling forms joints that isolate prisms perpendicular to the gradient of temperature. In granitic rocks, joints appear to be related to the relaxation of the vertical stress during erosion and exhumation.

IV-1-2 TENSION FRACTURES Tension fractures (also called tension gashes) are filled fractures with displacement usually perpendicular (or at high angle) to the fracture plane. Analysis of fibbers of infilling minerals gives information about the strain history related to the formation of the tension fracture. Tension fractures may formed "en echelon" system.

σ1 σ1 σ3 10 cm to 100 km

σ3 σ3 σ1 σ3 CHAPTER IV : DISCONTINUOUS DEFORMATION: STRUCTURES, INTERPRETATIONS

IV-1 STRUCTURES IV-1-2 TENSION FRACTURES

σ1 σ3

Gash fractures form an "en echelon" array along conjugated shear zones

Ductile shearing along the conjugated shear zones rotates the central part of the fractures. Fractures formed at different times during the shearing show different amounts of rotation, and the youngest may not be rotated at all.

Homogeneous simple shear Heterogeneous simple shear CHAPTER IV : DISCONTINUOUS DEFORMATION: STRUCTURES, INTERPRETATIONS

IV-1 STRUCTURES IV-1-3 STYLOLITIC JOINTS

In calcareous rocks and quartzite, dissolution along surface perpendicular to σ1, lead to a characteristic saw-tooth profile and an interdigitating cone-like form in three dimension known as stylolites. Styloliths result from dissolution under high stress of relatively soluble material (calcite, quartz) and the accumulation of relatively insoluble components (clay, phyllosilicates, carbon, iron, ores, etc.). σ1

σ1

σ1 IV-1-4 FAULTS Fracture with displacement parallel to the fracture plane. For an inclined fault, we call hanging wall the bottom surface of the upper fault block (hanging wall block) and footwall the top surface of the lower fault block (hanging wall block).

σ1 σ2 Normal sinitral Normal

σ3

σ2 σ1 σ3 Sinistral Rotational σ3 σ2 Reverse σ1

σ1 Listric σ2 σ3 Dextral CHAPTER IV : DISCONTINUOUS DEFORMATION: STRUCTURES, INTERPRETATIONS

IV-1 STRUCTURES IV-1-4 FAULTS Components of the displacement vector .

L T D L: Lateral horizontal displacement V // to the strike T: Transverse horizontal displacement // to the dip direction V: Vertical offset

D = T + V+ L

Movement plane: The movement plane is a plane perpendicular to the fault plane, and parallel to the striae (in purple on the stekch below). The movement plane contains the maximum (σ1) and minimum (σ3) axes of the stress ellipsoid.

M1 σ1 M 2

σ3

Fault plane 3 M S tr l i in a e e a o ti r o s n l ic k e n s id e CHAPTER IV : DISCONTINUOUS DEFORMATION: STRUCTURES, INTERPRETATIONS

IV-2 ORIENTATION OF THE AXES OF THE PALEOSTRESS ELLIPSOID During a tectonic event, fractures, stylolites and faults occur in close association. They offer key information on the orientation of the principal stresses axes and can also give information about the orientation of principal strain axes.

σ1 σ1

σ σ3 3

σ3 σ3

Map view σ1

σ1 Local perturbation of the state of stress around a fault zone

σ1

σ σ σ3 3 σ3 3

σ1 σ1 CHAPTER IV : DISCONTINUOUS DEFORMATION: STRUCTURES, INTERPRETATIONS

IV-2 ORIENTATION OF THE AXES OF THE PALEOSTRESS ELLIPSOID

σ3

σ1

σ2

σ1

σ3

σ2

σ3

σ1 CHAPTER IV : DISCONTINUOUS DEFORMATION: STRUCTURES, INTERPRETATIONS

IV-3 ORIENTATION OF THE AXES OF THE FINITE STRAIN ELLIPSOID

Find the direction of maximum shortening, and the direction of maximum lengthening

λ3 λ3

λ λ1 1

λ1 λ1

λ3

λ1

λ3

λ2

λ3

λ1

λ2

λ1

λ3

M 2 1 M

3 M CHAPTER IV : DISCONTINUOUS DEFORMATION: STRUCTURES, INTERPRETATIONS

IV-4 CHARACTERIZATION OF THE FINITE STRAIN ELLIPSOID Two directions of stretching => pan-cacke like ellipsoid Two directions of shortening => cigar like ellipsoid One invariant direction (direction of non-deformation)=> plane strain ellipsoid

Uniaxial prolate 2

K=1 8 n 1

s i

a l tr a s

n e

o n i a t l

c P i r t

1 s 0

K = 8

Flattening strain 0 K = 0

0 1 2 Ln (Y/Z) Ln (λ2/λ3) Uniaxial oblate

λ3 λ3 λ2

λ2≈ λ3 λ2≈ λ1 λ3

λ1 λ1 λ1

Uniaxial oblate Uniaxial prolate Plane strain CHAPTER IV : DISCONTINUOUS DEFORMATION: STRUCTURES, INTERPRETATIONS

IV-5 SHEAR CRITERIA

Tension gashes

Riedel fractures

Smooth surfaces

Mineral steps

Rough surfaces CHAPTER IV : DISCONTINUOUS DEFORMATION: STRUCTURES, INTERPRETATIONS

IV-5 SHEAR CRITERIA

• Rough, and smooth facets •Tectonics tool marks • Riedel shear S tr l i in a e e •Tension gash a o ti r o s n l ic k Minerals steps e • n s id e

Gash fracture Tool mark Mineral step

stylolites

Riedel shear fracture

Rough facet, or mineral step Smooth facet, or stylolite

Extension Compression

Riedel shear fracture CHAPTER IV : DISCONTINUOUS DEFORMATION: STRUCTURES, INTERPRETATIONS

IV-6 STRAIN REGIME

500 m

10 cm