Ductile Shear Zone ( 韌性剪切帶 ), Textures, and Transposition ( 移置作用 )

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1 Ductile Shear Zone ( 韌性剪切帶 ), Textures, and Transposition ( 移置作用 ) Moine Thrust in Scotland b/participants/dutch/vtrips/scot75may25-28.htm ages/geological/structural/mylonites/my lonite.jpg Jyr-Ching Hu, Department of Geosciences National Taiwan University

2 Ductile Shear Zone A tabular band of definable width in which there is considerably higher strain than in the surrounding rock. The total strain within a shear zone typically has a large component of simple shear( 簡單剪切 ), where rocks on one side of the zone are displaced relative to those on the other side. In its ideal form, a shear zone is bounded by two parallel boundaries, outside of which there is no strain. In real examples, shear zone boundaries are gradational.

3 Ductile Shear Zone The adjective ductile is used because the strain accumulates by ductile process, which range from cataclasis ( 破碎作用 ) to crystal-plasticity ( 晶體塑性流動 ) to diffusion. A shear zone is like a fault in the sense that it accumulates relative displacement of rock bodies, but unlike a fault, displacement in a ductile deformation mechanisms and no throughgoing fracture is formed. The absence of a single fracture is a consequence of movement under relatively high temperature conditions or low strain rates.

4 Crustal strength Sibson-Scholz fault model Brittle process and cataclastic flow Brittle-plastic transition 450 o c Geothermal Gradient of 20 o C/km-30 o C/km

5 Change in fault character with depth for a steeply dipping fault

6 Changes in the deformation behavior of quartz aggregates with depth

7 Distribution of the main types of fault rocks with the depth in the crust

8 Synoptic model of a shear zone

9 Brittle process, cataclastic flow, frictional regime and plastic regime Brittle process ( 脆性过程 ): Occur along the discontinuity in the few kms below Earth s surface which result in earthquakes if the frictional resistance ( 摩擦阻力 ) on discrete fracture planes is overcome abruptly. Cataclastic flow ( 破裂與碎屑流 ): A ductile process that displacement occurs by movement on many small fractures. Frictional regime ( 摩擦區 ): Frictional processes dominate the deformation at upper levels of discontinuity and this crustal segment. This region is pressure sensitive.

10 Brittle process, cataclastic flow, frictional regime and plastic regime Plastic regime ( 塑性區 ):: With depth, crystal-plastic and diffusional processes such as recrystallization and super-plastic creep, become increasing important due to increase of temperature. Below a depth of km for normal geothermal gradients (20 o C/km-30 o C/km) in Qtz-dominated rocks. Deformation in plastic regime is mostly temperature sensitive.

11 Frictional-plastic ( 摩擦 - 塑性過渡帶 ) and brittle-plastic transitions ( 脆性 - 塑性過渡帶 ) Frictional-plastic transition or brittle-plastic transition: transition zone between a dominantly frictional and dominantly plastic regime.

12 Brittle-plastic transition ( 脆性 - 塑性過渡帶 ) and brittle-ductile transition ( 脆性 - 韌性過渡帶 ) Brittle-ductile transition is in common use, it is technically not correct, because ductile processes (such as cataclasis) may occur in the frictional regime.

13 Mylonites ( 糜稜岩 ) Rigid clasts of varied lithologies in a fine-grained, crystal-plastically deformed marble matrix. (Grenville Orogen, Ontario, Canada)

14 Mylonites ( 糜稜岩 ) A fault rock type with a relatively fine grain size as compared to the host rock and resulting from crystal-plastic processes. Dynamic recrystallization occurs at different temperatures Calcite 250 o C Quartz 300 o C Feldspar 450 o C

15 Types of Mylonites Mylonite: 50-90% matrix Protomylonite ( 原生糜棱岩 ): < 50% matrix Ultramylonite ( 超糜棱岩 ): % matrix Blastomylonite ( 再晶糜稜岩 )(blastos meaning growth) and clastomylonite ( 碎斑糜棱岩 ) (klastos meaning broken): Describe mylonites containing large grains surrounded by a fine-grained matrix and grew during mylonitization or remained from original rock.

16 Shear-sense indicators Ductile shear zones concentrate displacement at deeper levels in the crust, where recognized markers that determine offset are often absent. Sense of displacement: describes the relative motion of opposite sides of the zone (left-lateral or right-lateral). Magnitude of displacement: distance over which one side moves relative to the other.

17 Plane of Observation Mylonitic foliation

18 Internal reference frame Most mylonites contains at least one foliation and lineation which we use as an internal reference In the field we look for outcrop surfaces (or cut an oriented sample in the lab) that are perpendicular to mylonitic foliation and parallel to the lineation. We assume that the lineation coincides with the movement direction of the shear zone.

19 Plane of Observation The displacement sense is the same in geographic coordinates, it is a good habit to analyze surfaces in the same orientation. From the opposite side: Left-lateral, why?

20 Types of Shear-sense indicators (1) Grain-tail complexes (2) Disrupted grains (3) Foliations (4) Textures (or crystallographic fabrics) (5) Folds

21 Grain-Tail Complexes A K-feldspar clast with a tail of fine-grained plagioclase of the -type complex (California, USA)

22 Grain-Tail Complexes: Rotated porphyroblasts ( 旋轉斑晶系 ) -type: characterized by wedge-shaped tails that do not cross the reference plane when tracing the tail away from the grain Rotate the Greek letter over 90 o -type: the tail wraps around the grain such that if cross cuts reference plane when tracing the tail away from the grain

23 Snowball garnet

24 Diagnostic forms of porphyroblasts S e : solid lines, external foliation S i : dashed lines, internal foliation

25 Progressive development of snowball textures How do we preserve a spiral pattern in garnet and what can it tell us? Metamorphism is synkinematic. Assignment: Reading 13.4 Deformation and metamorphism

26 Evolution of a -type complex to -type grain-tail complex Mixed occurrence of -type complex to -type: Rate of recrystallization or neocrystallization and rotation of grain 1. Rail formation is fast relative to rotation: -type 2. The rotation of grain, the tail is dragged along and wrap around the grain : - type

27 Fractured Grains and Mica Fish Synthetic fractures (bookshelf-type or domino-type): Fractures oriented at low angles to the mylonitic foliation have a displacement sense that is consistent with the overall shear sense of the zone.

28 Fractured Grains and Mica Fish Antithetic fractures: Fractures at angles greater than 45 o to the mylonitic foliation show an opposite sense of movement.

29 Formation of mica fish: Fish flash 1. Large phyllosilicate grains: Mica and biotite in quartzo-feldspathic rocks and phlogopite in marbles 2. Micas are connected by a mylonitic foliation and their basal planes (0001) oriented at an oblique angle to mylonitic foliation Stair-stepping geometry Basal planes of mica

30 Characteristic geometry of C-S and C-C structures in a dextral shear zone 1. Most mylonites show at least one well-developed foliations at low angle to the boundary of shear zone. 2. S-foliation: S comes from French word for foliation, schistosité. 3. C-foliation: C comes from French word for shear, cisaillement.

31 Characteristic geometry of C-S and C-C structures in a dextral shear zone 3. C -foliation: Discrete shear displacement that is oblique to the shear zone boundary.

32 Summary diagram of shearsense indicators

33 Strain in Shear Zone: Rotated Grains Snowball granets: -type grain-tail complexes; in particular the mineral garnet show this behavior, in which trapped matrix grains eventually produce a spiraling trails. = tan = = tan = : mechanical coupling between matrix and grain Analog Experiment

34 Strain in Shear Zone: Rotated Grains =1, full coupling (clean ball bearing) =0, no coupling 0< <1, partial coupling (greasy ball bearing) = tan = =0.5: Assume that grain rotate in a viscous (Newtonian) fluid, considerable slippage occur at contact between matrix and grain. Analog Experiment

35 Homogeneous and Heterogeneous strain in shear zone

36 Deflection of the mylonitic foliation Drenville Orogen, Ontario Canda. Width of view is ~20 cm.

37 Strain in Shear Zone: Deflected foliations Shear zone is characterized by a mylonitic foliation (Sfoliation) that is at ~45 o to the shear-zone boundary = 2/tan2 Progressive simple shear Angular relationship ( ) between foliation and shear-zone boundary, and shear strain Wk = 1 Kinematic vorticity number Prefect shear zone

38 Strain in Shear Zone: Deflected foliations General shear with a shortening component is called transperssion and an extensional component is called transtension Component of pure shear Nonperfect simple shear (or general shear)

39 Non-commutative nature of strain tensor Superimposing simple shear on pure shear Superimposing pure shear on simple shear Simultaneously adding simple and pure shear

40 C-axis Development of a crystallographic-preferred orientation by dislocation glide: : angle of shear along glide plane : rotation angle of material line BC ABCD: crystallographic glide planes : rotation angle of the c-axis with respect to an external ref. system : angle of finite extension axis

41 The Symmetry Principle: Curie Principle Orthorhomic: 3 two-fold axes or 3 symmetry planes Coaxial strain: Incremental and finite strain ellipsoids differ only in shape, not in orientation cube Pierre Curie Monoclinic: 1 two-fold axis or 1 symmetry plane cube

Relationship between shape, crystallographic, S and C When shearing the aggregate, a pattern emerges in which the majority of c-axes rotate toward an orientation perpendicular to

42 Relationship between shape, crystallographic, S and C When shearing the aggregate, a pattern emerges in which the majority of c-axes rotate toward an orientation perpendicular to the bulk shear plane. A dimensional-preferred fabric is formed that define the mylonitic foliation (S-foliation). S: mylonitic foliation C: shear plane Randomly oriented Simple shear

43 Foliation in shear zone and associated crystallographic fabrics C-axis girdle S-foliation deflection: Angular relationship between S and C decreases with increasing shear strain (S and C approach parallelism). C-axis girdle Corresponding a-axis patterns show n change.

44 Asymmetric c-axis fabrics Reference: Mylonitic foliation S E-twinning dominate calcite deformation Basal slip occurred

45 Fold Transposition in a layer rock that undergoes non-coaxial, layer-parallel displacement With increasing shear, the oblique (short) limb of the asymmetric fold rotates back into a foliation-parallel orientation. Foliation-parallel shear

46 Fold Transposition in a layer rock that undergoes non-coaxial, layer-parallel displacement The resulting perturbation gives rise to a new fold that is superimposed on the original structure. Continued shear reorients the fold pattern back into a layer reorients the fold pattern back into a layer-parallel orientation. Foliation-parallel shear

47 Fold Transposition Highlight two aspects of folds in shear zone: (1) Fold symmetry may be representative for the sense of shear: Z-vergence: Right-lateral shear zone. S-vergence: Left-lateral shear zone A possibility, not a rule. At high sear strains the vergence of small folds may actually reverse.

48 Fold Transposition Highlight two aspects of folds in shear zone: (2) Folding is a progressive process, resulting in complex patterns of folding and refolding. Fold transposition occurs at all scales, from microfolds to kilometer-scale folds.

49 Transposed mafic layer in granitic gneiss: Snake outcrop Mafic (dark) layer is traced as a single bed refolded numerous times

50 Reversal in fold vergence (from S-shape to Zshape ) with increasing shear strain in a rightlateral shear zone Z-vergence: Right-lateral shear zone. S-vergence: Left-lateral shear zone.

51 Fold Transposition: Competent layer Folds in areas of high strain are often disrupted, preserving only isolated hinges or fold hooks. Progressive shortening: thinning of limbs And locally hinges become detached. Are there criteria to recognize transposition? Clues: 1. Regular repetition of lithologies; 2. Parallel between foliation and bedding; 3. Occurrence of minor isoclinal folds and fold hooks. Fold hooks boudinage

52 An example of an early stage of transposition Newfoundland, Canada

53 Sheath Folds ( 劍鞘形褶皺 ) Restricted to regions of high shear, it can define shear sense in ductile shear zones. A special type of double-plunging folds ( 雙傾伏褶皺 ), where the hinge line is bent around by as much as 180 o. Layering in a sheath fold is everywhere at a high angle to the profile plane ( 正交剖面 ), which give the characteristic eye-shaped outcrop pattern.

54 Sheath Folds ( 劍鞘形褶皺 ) Formed when the hinge line ( 樞紐線 ) of a fold rotates passively into the direction of shear, while the axial surface ( 褶皺軸面 ) rotates toward the shear plane. The location of nose of sheath folds points in the direction of movement, but this can be determined only when the folds are fully exposed. Most commonly, sheath folds define the direction of shear rather than shear sense, with the hinge line approximately parallel to the shear direction.

55 Conical geometry of a sheath folds Stretching lineation Shear plane Lowest amount of shear at the left Shear direction highest shear strain at the right Hinge line measurements Lower-hemisphere projections

56 Assignment 1 Reading: Drawing and explain the Figure Summary diagram of shear sense indicators in a sinistral shear zone

57 Assignment 2 Structural Analysis: An interactive course for Earth Science Student by Declan G. De Paor Chapter 14: shear sense indicators (1) Offset markers; (2) Riedel shears; (3) Domino fault; (4) Inclusion trail; (5) grains; (6) grains; (7) Mica fish; (8) Sheath folds; (9) Asymmetric folding; (9) Bedding/foliation; (10) Restraining bends; (11) Releasing bends; (12) Terminations; (13) En echelon array; (14) S-C foliation

CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION

CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION Va-1 INTRODUCTION Heterogeneous deformation results from mechanical instabilities (folding and boudinage) within an heterogeneous material or from strain localization in an homogeneous material (shear