Fabrics ( 組構 ): Foliation and Lineations ( 葉理與線理 )

Transcription

1 Fabrics ( 組構 ): Foliation and Lineations ( 葉理與線理 ) 台灣板岩地質區板岩的結構, 呈現劈理折射的現象 楊昭男教授提供 Jyr-Ching Hu, Department of Geosciences National Taiwan University

2 Fabric Terminology Random fabric ( 散亂的組構 ): No preferred orientation (i.e., alignment) of the fabric element. Nonrandom fabric (preferred fabric, 有優向性的組構 ): Fabric elements are aligned in some manner and/or repeated at approximately regular spacing. Planar fabric (foliation, 葉理 ): Fabric element is a planar or tabular feature. Linear fabric (Lineation, 線理 ): Fabric element is a linear feature.

3 Basic categories of fabrics Random fabric A 1-D preferred fabric A foliation A lineation

4 Distinction between continuous and spaced fabrics Continuous fabrics ( 連續性的組構 ) Fabric elements are closer than 1 mm (below the resolution of the eye) Spaced fabrics ( 間隔性的組構 )

5 Continuous fabric ( 連續的組構 ) 若我們把岩石切成一片片的薄片, 當薄片的厚度薄到組成岩石的顆粒的尺寸時, 仍然可以鑑定出組織元素的方向時, 則我們稱這塊岩石裡面的組構元素是連續的 在實際的操作上, 若組織元素緊密到 1 mm 以下, 我們的肉眼無法看出組構元素的間隔時, 我們就稱這塊岩石裡面的組構元素是連續的, 或者這塊岩石具有連續的組構 Spaced fabric ( 間隔的組織 ) ( 組織元素間隔出現的組織 ) 當我們的肉眼很明顯地可以看出組構元素的間隔時, 我們就稱這塊岩石裡面的組構元素是間隔的, 或者這塊岩石具有間隔的組構

6 The nature of tectonites ( 構造岩 ) Tectonites: Rock with a penetrative tectonic fabric ( 構造組織發展得很透徹的岩石 ) L-tectonite: Linear fabric elements dominate; S-tectonite: A rock with dominantly planar fabrics; LS-tectonite: Rock with both types of fabric elements S-tectonite L-tectonite LS-tectonite

7 Tectonites ( 構造岩 ): 所謂的構造岩指的是構造組織發展得很透徹的岩石 (Rocks with penetrative tectonic fabric). S-Tectonite: 透徹地具有葉理的構造岩 L-Tectonite: 透徹地具有線理的構造岩 LS-Tectonite: 透徹地具有葉理與線理的構造岩 Penetrative( 透徹地 ): 在一塊岩石當中, 我們若隨意地採取樣本, 都可以採集到某種組織元素時, 則我們稱這塊岩石透徹地具有這種組織元素

8 Foliations ( 葉理 ) Foliation: Any type of planar fabric in a rock. Bedding, cleavage ( 劈理 ), schistosity ( 片理 ) and gneissosity ( 片麻葉理 ) all qualify as foliations. Fractures are not considered to be foliations, because fractures are breaks through a rock and are not a part of rock itself. Convention: Track of different foliations (s 0, s 1, s 2, ) S 0 : bedding; S 1 : First foliation formed after bedding; S 2 : Foliation forms after S 1. L: Lineation, S: surface in English, schistosité in French or Schieferung in German

9 由於葉理 (foliation) 的定義相當分歧, 造成相當多的困擾與不必要的誤解, 因此目前有一種趨勢, 將岩石當中所呈現的面狀構造統稱為葉理, 再將葉理分為原生的葉理 (primary foliation) 與次生的葉理 (secondary foliation) 原生葉理指的是岩石形成時所形成的面狀構造, 而次生葉理指的是岩石形成之後受到變形作用所產生的面狀構造

10 What is cleavage ( 劈理 )? Nongenetic definition: A secondary fabric element, formed under low-temperature conditions, that imparts on the a rock a tendency to split along planes. 1. Feature of rock that forms subsequent to the origin of rock. 2. Tectonic planar fabrics formed at or below lower greenschist facies conditions (i.e., 300 o C). 3. The term cleavage is not used when referring to the fabric in schists or in gneiss. 4. A rock with cleavage, there are planes of weakness across which the rock may break when uplifted and exposed at the surface, even though cleavage itself forms without loss of cohesion. Closely spaced fractures array is not a cleavage.

11 劈理的定義重點在於 : 1. 劈理是在岩石形成之後受到變形作用才產生的現象 2. 劈理是在變質作用中變質相不超過綠片岩相 (lower greenschist facies) 的低溫狀態 ( C) 下形成的 3. 當岩石具有劈理構造時, 很容易沿著劈理裂開, 即使劈理面形成時, 劈裡面之間的內聚力並未失去 因此排列得很緊密的破裂面並不是劈理, 因為破裂面形成時, 破裂面之間的內聚力已被破壞掉

12 Slaty cleavage 台灣板岩地質區板岩的結構, 劈理種類為板劈理 Courtesy of Professor Yang Chao-Nan, Dept. Geosciences, NTU

13 劈理的領域化特性 Powell (1979) 經過詳細的觀察以後, 首先認為在各種的觀察尺度上, 有劈理發育的岩石都可以分辨出兩種礦物的組成與結構都不相同的領域構造 (domainal structures): 劈理領域 (cleavage domain): 劈理面較為密集的領域 小岩領域 (microlithon domain): 劈理面較為不密集的領域

14 雲母片岩在顯微鏡底下所呈現的領域構造 (domainal structures) 劈理領域 小岩領域 劈理領域

15 Spaced disjunctive cleavage ( 無前理劈理, 分離式劈理, 無夾皺劈理 ): solution cleavage in limestone 小岩領域 劈理域

16 Disjunctive cleavage ( 無前理劈理 ) The disjunctive implies that the cleavage domain cut across a preexisting foliation in the rock (bedding) without affecting the orientation of the preexisting foliation in the microlithons. Pressure solution is always involved in the formation of a disjunctive cleavage, pressure-solution cleavage ( 壓溶劈理, 溶解劈理 ) and stylolitic cleavage ( 縫合線劈理 ) have been used for this structure.

17 無夾皺劈理 (disjunctive cleavage) 容易裂開的劈理領域為泥質或碳酸鈣物質所組成 劈理領域中容易裂開的劈理面類似破裂面, 其紋路形狀有平直的 脈絡狀 犬牙交錯狀的縫合線 這種劈理大多形成於未經過變質作用的石灰岩或泥灰岩 (marls) 中

18 Stylolite ( 縫合線構造 ) Courtesy of Professor Yang Chao-Nan, Dept. Geosciences, NTU

19 Pressure shadows ( 壓影構造 ) 顯微鏡底下磁鐵礦的壓影構造 Courtesy of Professor Yang Chao-Nan, Dept. Geosciences, NTU

20 Cleavage description and classification Sutured domains Planar domains Wavy domains Anastomosing array of wavy domains Spaced cleavage: Domain spacing

21 Anastomosing array of wavy domains 台灣板岩地質區變質砂岩的結構, 劈理種類為脈絡形間隔劈理 Courtesy of Professor Yang Chao-Nan, Dept. Geosciences, NTU

22 Pencil Cleavage Fine-grained sedimentary rock (shale or mudstone) breaks into elongate pencil-like shards because of its internal fabric. Pencils are 5-10 cm long and 0,5-1 cm in width. It results from the interaction of two fracture sets. Pencil cleavage forms because of the strong shape anisotropy of clay flakes creates a preferred orientation parallel to bedding.

23 Progressive development of slaty cleavage via the formation of pencil structure Cleavage formation process begin to take place: Large detrital phyllosilicates fold and rotate, while fine grain undergo pressure solution along domain perpendicular to the shortening direction, and new clay crystallizes. Layer-parallel shortening Strain ellipsoid: pancake

24 板劈理的階段性發育 (a) 沈積物沈積成為沈積岩岩層 (b) 在成岩作用階段, 沈積岩岩層受到上覆岩層向下壓的壓縮作用, 沈積物顆粒間的水分被排除, 沈積物顆粒被壓得更為緊密, 使岩層的體積變小, 而片狀的礦物也因此被旋轉成大致要平行層面的方向, 形成比較粗糙的葉理 這種平行層面的層面葉理稱為 diagenetic foliation 或層面劈理

25 板劈理的階段性發育 (c) diagenetic foliation 形成之後, 岩層受到大地壓縮作用, 形成與層面斜交的葉理 這與層面斜交的葉理和與層面平行的葉理相交, 形成交線線理 具有這種交線線理的劈理, 稱為鉛筆劈理 (pencil cleavage) (d) 隨著岩層整體應變的增加, 先前形成的鉛筆劈理中, 與層面斜交的劈理面發育得更為平整, 而形成板劈理

26 Slaty cleavage ( 板劈理 ) Slaty cleavage is defined by strong dimensionally preferred orientation of phyllosilicates ( 頁矽酸鹽礦物 ) in a very clay-rich rock. Slaty cleavage forms under temperature condition that 三閏 mark the onset of metamorphism (250 o C-350 o C), there is a notable decreasing in the amount of interlayered water in clays; that is, smectite (, water-bearing clay) transforms to illite ( 伊來石 ). 在泥質岩石當中, 片狀黏土礦物很強烈地平行排列 板劈理中的黏土礦物主要為由膨潤石 (smectite) 轉變而成的依來石 (illite)

27 Phyllitic cleavage ( 千枚岩劈理 ) and schistosity ( 片理 ) When metamorphic conditions reach lower greenschist ( 低綠色片岩相 ), the clay and illite in a pelitic rock ( 泥質的岩石 ) react to form white mica (muscovite, 白雲母 ) and chlorite ( 綠泥石 ). Phyllite ( 千枚岩 ): Rock is composed of strongly aligned fine-grained white mica and/or chlorite. Phyllitic cleavage is intermediate between slaty cleavage and schistosity. Schist ( 片岩 ): When metamorphic conditions get into the middle greenschist facies ( 中綠色片岩相 ),, the minerals in a pelitic lithology react to form coarser-grained mica and other minerals.

28 Phyllitic cleavage ( 千枚岩劈理 ) and schistosity ( 片理 ) Conveniently, a schist is named by the assemblage of metamorphic index minerals that in contains, garnetbiotite schist ( 石榴子石 - 黑雲母片岩 ). Phyllite: Rock is composed of strongly aligned finegrained white mica and/or chlorite. In schist contains porphyroclasts ( 殘碎斑晶, relict large crystals) or porphyroblasts ( 變斑晶或斑狀變晶 ) newly grown large crystals), the schistosity tends to be wavy, as the mica curve around the large crystals.

29

30 千枚岩劈理 (Phyllitic cleavage) 在低度的綠色片岩變質相下, 泥質岩石中的黏土礦物與依來石轉變成白色雲母與綠泥石 在礦物轉變的過程中, 這些白色雲母與綠泥石受到不均勻應力作用而作平行的排列因而形成葉理時, 這種葉理就稱為千枚岩劈理 片理 (Schistosity) 在中度的綠色片岩變質相下, 泥質岩石中的白色雲母與綠泥石轉變成顆粒較粗的雲母礦物與其他礦物 在礦物轉變的過程中, 這些雲母礦物與其他礦物受到不均勻應力作用而作平行的排列因而形成葉理時, 這種葉理就稱為片理

31 大南澳片岩的片理的顯微照片 Courtesy of Professor Yang Chao-Nan, Dept. Geosciences, NTU

32 Continuous cleavage

33 Crenulation Cleavage ( 夾皺劈理 ) A lithology containing a closely and evenly spaced foliation that is shortened in a direction at a low angle of this foliation. Symmetric crenulation cleavage asymmetric (sigmoidal) crenulation cleavage

34 Crenulation cleavage ( 夾皺劈理 )

35 大南澳片岩夾皺片理的顯微照片 Courtesy of Professor Yang Chao-Nan, Dept. Geosciences, NTU

36 Differentiation ( 礦物分異作用 )during the formation of crenulation cleavage Crenulation cleavage forms under conditions that are also amenable to the occurrence of pressure solution ( 壓溶作用 ). Qtz: Removed from limbs of the microfold and precipitates in the hinges as the crenulation cleavage forms.

37 Gneissic layering and Migmatization ( 片麻岩層理與混合作用 ) Foliated gneiss (gneissosity): metamorphic rock in which the foliation is defined by compositional banding( 成分條帶 ). Light-colored layers: feldspar and Qtz Dark-colored layers: amphibole ( 角閃石 )/pyroxene ( 輝石 )(and/or biotite) Gnesis formation (amphibolite to granulite facies, 角閃石相到粒變岩相 ): muscovite reacts to form feldspar, so the rocks contains no schistosity.

38 白沙橋底下之河床上, 由上游往下游看, 在相片中央, 可見片麻岩岩體內部大理岩與片麻岩之界面被褶皺成小型的等斜褶皺, 形成大理岩似以尖釘狀插入片麻岩中的景象

39 Fold hinges in transposed ( 移置作用 ) gneiss: 層理被葉理所置換 (transposed) 的片麻岩中偶可觀察到殘留下來的褶皺軸部 Felsic mineral ( 石英質 ): feldspar, Qtz Mafic mineral ( 基質, 含鐵鎂的 ): amphibole/pyroxene, biotite

40 Gneissic layering and Migmatization ( 片麻岩層理與混合作用 ) Paragneiss ( 副片麻岩 ): Sedimentary protolith ( 由沈積岩變質而成的片麻岩 ). Orthogneiss ( 正片麻岩 ): Igneous protolith ( 由火成岩變質而成的片麻岩 ). Augen gneiss ( 眼球片麻岩 ): Contains relatively large feldspar clasts floating in a fine-grained matrix.

41 Augen Gneiss ( 眼球片麻岩 )

42 片麻岩 (gneiss) 片麻岩是一種葉理為成分條帶構造 (compositional banding) 的變質岩 所謂的成分條帶構造是淡色的條帶與暗色的條帶交互出現的構造 通常, 淡色條帶的礦物成分主要為長石與石英, 而淡色條帶的礦物成分主要為輝石 角閃石或黑雲母 這種條帶構造稱為片麻葉理 (gneissosity) 由於片麻岩是在比較高度的變質環境中形成的 ( 角閃岩變質相到變粒岩相 )(amphibolite to granulite facies), 原有的白雲母已經變化成長石, 因此不會形成片理 由沈積岩變質而成的片麻岩稱為副片麻岩 (paragneiss) 由火成岩變質而成的片麻岩稱為正片麻岩 (orthogneiss)

43 片麻岩構造 (gneissic structure) 沈積岩或火成岩在中 高度變質作用時, 經過再結晶作用所發展出來的一種條帶形葉理 這種變質岩主要是由中粒到粗粒的礦物所組成 成份或顆粒大小不同的礦物常常分別聚集在一起而呈現成分不同的條帶構造 (compositional banding), 而片狀的 板狀的或長條狀的礦物則排成平面構造且呈現方向性的排列, 透鏡形與顆粒形的礦物則大致呈平行的排列

44 Mechanism of formation of a gneiss (a) Inheritance from an original lithology (b) Creation of new composition banding via transposition: Hinge of fold detached (c) Metamorphic differentiation: Diffusion, ions to be excluded (d) Igneous process: Litpar-lit intrusion

45 片麻岩成分條帶 (compositional layering) 的形成機制 (a) 原岩為砂岩與泥岩, 砂岩變質成為石英岩, 泥岩變質為雲母片岩 (b) 原岩的層狀構造經由岩層置換作用 (transposition) 形成成分不同的條帶構造 (c) 變質分異作用 (metamorphic differentiation) 造成白色的長石石英富集帶與黑色的鐵鎂礦物富集帶 (d) 部分熔化作用產生的岩漿沿著弱面侵入變質岩, 形成一層層的侵入岩構造 (lit-par-lit intrusion)

46 Migmatite ( 混合岩 ) The metamorphic rocks exposed in this outcrop near Michigamme, Michigan are over 3 billion years old. Eric Cohen, Westhampton Beach High School

47 Migmatite ( 混合岩 ) 當變質溫度很高時, 岩石會開始熔化, 但由於岩石是由不同的的礦物組成而成的, 不同的礦物具有不同的熔點, 因此熔點較低的礦物 ( 例如石英 雲母與鈉質斜長石 ) 會先熔化而成為液體 ( 岩漿 ), 而熔點較高的礦物 ( 例如橄欖石 輝石 角閃石與鈣質斜長石 ) 仍然維持固體的狀態 (refractory minerals) 這種只有部分的岩石熔化的作用稱為部分熔化作用 (partial melting or anatexis) 進行部分熔化作用的岩石就像土石流一樣, 未熔化的礦物或岩體會懸浮在黏稠的岩漿當中 當這些混雜在一起的固體與液體受到擠壓時, 就會像土石流一樣地流動 變形, 形成褶皺與再褶皺的構造 當溫度下降後, 這種混合著酸性岩漿與基性岩塊的混合物就固化成為所謂的混合岩

48 Cleavage and Strain Does the study of a foliation in the field provide any constraints on the nature of strain in the region? A whisy-washy answer: sometimes yes, sometimes no. Strain marks in a cleaved rock

49 Strain markers: Reduction spots

50 Strain markers: Reduction spots The rock has a light colored reduction spot. When the rock forming sediments were deposited there was a small bit of organic material mixed in. As this was decomposed by bacterial processes the Fe 3+ was reduced to Fe 2+. Reduced Iron is much more soluble in water therefore subsequent water flow through the rock rinsed away the iron. The redish color in the rest of the rock is the oxidized iron mineral hematite that is present in the cement.

51 The lose or not to lose volume Assuming no change in the third dimension, the intermediate strain axis Y=1 Volume loss strain Volume constant strain

52 Graptolite: strain marker for finite strain Studies of deformed markers and geochemical studies: Rock volume may have decreased by up to 50% during cleavage formation

53 Geochemical studies Based on relative immobile elements TiO 2, Y, and Zr ~45%

54 Foliations in Folds and Fault Zones Pencil cleavage How cleavage develops during progressive deformation? Cleavage refraction: Change in cleavage attitude that occurs where cleavage domain cross from one lithology into another of different competency. Layer-parallel shortening Region shear Flexural slip Disjunctive cleavage

55 台灣板岩地質區板岩的結構, 呈現劈理折射的現象 Courtesy of Professor Yang Chao-Nan, Dept. Geosciences, NTU

56 Transecting Cleavage Cross-cutting cleavage forms in transpressional environments ( 橫移壓縮 : components of both pure and simple shear). Q: Is it subjected to dextral shear of sinistral shear? Dextral shear (dextral transpression): Counterclockwise transecting cleavage relative to roughly synchronous fold hinges Sinistral shear: Clockwise transecting cleavage.

57 台灣變質岩區的層理與葉理構造 ( 雙龍橋 ) Courtesy of Professor Yang Chao-Nan, Dept. Geosciences, NTU

58 Cleavage-bedding relationships and cleavage refraction Facing: younging

59 Assumption: S 1 cleavage is axial planar to the F 1 fold A: On the overturned limb of the upward facing part of the F 2 fold, bedding dips more steeply than cleavage.

60 Assumption: S 1 cleavage is axial planar to the F 1 fold B: On the upright limb of the upward facing part of the F 2 fold, cleavage dips more steeply than bedding.

61 Assumption: S 1 cleavage is axial planar to the F 1 fold C: On the upright limb of the downward facing part of the large fold, bedding dips more steeply than bedding.

62 Assumption: S 1 cleavage is axial planar to the F 1 fold D: On the overturned limb of the downward facing part of the large fold, cleavage is steeper.

63 中橫公路大禹嶺合歡隧道口林道

64 中橫公路大禹嶺合歡隧道口林道

65 Examples of Lineations a: Fold and crenulation hinges b: mullions ( 窗櫺構造 ) c: boudins, boudinage ( 串腸構造 )

66 Mullions ( 窗櫺構造 ) 尖瓣褶皺中硬層與軟層界面所形成的窗櫺構造 Cusplike corrugations that form at the contact between units of different competencies in a deformed multilayered sequences, the axes of mullions are a lineation. More rigid lithology occurs in convex bulge that protrude into the ductile lithology. Because of their mechanical origin, mullions cannot be used as a facing indicator.

67 Boudinage ( 串腸構造 )

68 Elongate pebbles in a stretched conglomerate ( 被拉長的礫岩中的長條形卵石 )

69 Surface Lineation Intersection lineation Slip lineations ( 斷層面上的滑動線理 ) Bedding-cleavage interaction 劈理面與層面交叉所呈現的線理 Groove lineations Fiber lineations

70 Shear wave splitting ( 剪力波分離 ) 若一均質物體 (isotropic) 其內部性質會隨著方向變化而改變, 此物體便具有非均向性 當剪切波經過非均向介質時將會分離為傳遞速度不同且粒子運動方向互相垂直的二波 : 傳遞速度較快者為快 S 波, 較慢者稱為慢 S 波 快 S 波的粒子運動方向稱為極化方向 (ψ), 代表非均向體內部物質排列的方向 ; 快波與慢波之間的延遲時間差 (δt) 則反映傳遞路徑內非均向性的強度或非均向層之厚度

71 Shear wave splitting Courtesy of Professor Rau Ruey-Juin Dept. Earth Sciences, NCKU

72 Magnetic fabrics in pressively cleavaged mudrock AMS: Anisotropy of magnetic susceptibility ( 磁感率不均向性 ) Maximum susceptibility axis: Minimum susceptibility axis: The minimum axes are sensitive Indicator of cleavage intensity

73 Assignments Structural Analysis: An interactive course for Earth Science Student by Declan G. De Paor Chapter 14: Fabrics (1) Foliation (2) Lineation (3) S-C foliation (4) Low/High T

74 Homeworks: Geometric Rules of bedding and cleavage 1. Foliations in folds and fault zone (p ) 2. If cleavage/foliation is axial planar the upright limb of an overturned fold will dip less than the fabric whereas the overturned limb will have a steeper dip. Please explain which is the possible structural interpretation in Figures B and C for the outcrop in Figure A. 楊昭男教授提供