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Lecture 6 Lines/Planes • Different Deformation Histories: Same Result? • the Difference Between Pure and Simple Shear • Stretching & Shortening

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Lecture 6 Lines/Planes • Different Deformation Histories: Same Result? • the Difference Between Pure and Simple Shear • Stretching & Shortening This lecture • Last lectures Methods of • Mohr circle for strain Structural Geology • This lecture • Look at deformation history of individual lecture 6 lines/planes • Different deformation histories: same result? • The difference between pure and simple shear • Stretching & shortening The Mohr circle for strain Vorticity • A line that makes an angle with the X-axis " • Vorticity is the average rotation of lines • Stretches by a factor L • Vorticity is strain dependent • And rotates by an angle ! • Here: 7.5° " a b% Fij = $ ' # c d & L ! 2" The X-axis plots on {a,-c} 7.5° The Y-axis plots on {d,b} ! Kinematic vorticity number: Wk Wk and type of strain • We need a "number" which tells us what the • Wk = 0 • 0 < Wk < 1 • Wk = 1 type of deformation is • Pure shear • General • Simple • And that is independent of strain: Wk shear shear W W = k R ! Finite strain ratio and area change Exercise # 2 "0.5& • Maximum (#1) and minimum (#2) stretch are • The position gradient tensor is: Fij = % ( points on circle furthest and closest from $ 0.25 0.7 ' origin " • Draw the Mohr circle R = 1 • How much do the X and Y axes stretch and rotate? f • What are R , !A, and W ? " f k 2 • What are the orientations of the finite stretching axes (FSAs) in the L + R undeformed state? " Rf = • Draw the box shown here: ! L # R • What does it look like in the deformed state? • In the undeformed and in the deformed state ! show the orientations that rotate to the left Area change (!A) and to the right A 1 ! " = #1#2 $ ! Exercise Exercise # 2 "0.5& # 2 "0.5& • The position gradient tensor is: Fij = % ( • The position gradient tensor is: Fij = % ( $ 0.25 0.7 ' $ 0.25 0.7 ' • Draw the Mohr circle • How much do the X and Y axes stretch and rotate? • X-axis: e = 2.02, $ = 7.0° ! (x) (x) ! • Y-axis: e(y) = 0.86, $(y) = 35.5° Exercise Exercise # 2 "0.5& • The position gradient tensor is: Fij = % ( • What are the orientations of the finite stretching axes $ 0.25 0.7 ' (FSAs) in the undeformed state? • What are Rf, !A, and Wk? • e1 makes angle of -26.2/2 = -13.1° with the X-axis Rf = 2.06/0.74 = 2.78 • And rotated 15.3° Wk = -0.366/0.662 = -!0 .55 !A = 2.06·0.74-1 = 0.52 (+52%) Exercise Different paths to same result • Draw the box shown here: • What does it look like in the deformed state? • In the undeformed and in the deformed state show the orientations that rotate to the left and to the right undeformed Simple shear Pure shear Rigid-body rotation Pure shear Pure shear + rotation • Pure shear deformation with FSA's parallel to X-Y • Adding rigid-body rotation of 26.6° brings • = 1.62 deformed x-axis back to horizontal (0° #1 rotation) Rf = 2.61 !A= 0 (no area change) • #2 = 0.62 • Rigid-body rotation means rotating all lines • X makes angle of 58.3° with x-axis by same amount: • Rotate Mohr circle around origin • The x-axes rotates 26.6° • Result = simple shear (or is it?) ? Simple shear = pure shear + rotation Pegmatite vein at Cap de Creus Stretch of line // x-axis ar she Pure Simple shear 2.62 • With pure shear at 58.3° to the x-axis • The line // x-axis first shortens and then stretches again • At Rf = 2.62 it has a stretch of exactly e(x) = 1 • Pegmatite vein cuts • With simple shear // x-axis main foliation (S01) • The line // x-axis does not stretch or shorten ever • Tourmaline rim present What are strain and kinematics? Simple shear // vein • Let us assume simple shear // vein • Vein does not stretch: e(v) = 1.0 • Vein does not rotate !(v) = 0° • Make a Mohr graph and plot the point vein representing the vein S01 • Pegmatite vein no stretch: e " 1.0 (v) vein • In tourmaline rim main foliation (S01) originally at 90° to vein • S01 at 25° to vein away from rim: rotated 65° clockwise Case 2: foliation does not rotate The Mohr circle for strain y-axis • A line that makes an angle " with the X-axis • Stretches by a factor L x-axis • And rotates by an angle ! • Now suppose the foliation did not rotate " a b% • Define foliation // x-axis Fij = $ ' • Draw the new Mohr circle # c d & L • What is the vorticity number? ! The X-axis plots on {a,-c} • What is the position gradient tensor? 2" The Y-axis plots on {d,b} • Is this scenario likely? • Consider the stretch history of the vein • (still assume no area change) ! Instantaneous stretching & Shortening, then stretching shortening: simple shear • The instantaneous stretching axes (ISA's) are at 45° to the shear plane • There a 4 quadrants • 2 of instantaneously shortening lines • 2 of instantaneously stretching lines Always shortened • The orientations of ISA's do not change during steady-state deformation • Lines rotate and may rotate from a • Some layers may appear both stretched and Shortened, shortening field into a stretching field then stretched shortened: folding AND boudinage Always stretched Instantaneous stretching & Now for pure shear shortening: simple shear • Draw two Mohr circles for progressive • Repeat the previous exercise for pure shear simple shear • Use as the position gradient tensor: • One for %=1 • One for % =2 " 2 0 % • Determine for both finite shear strains F = $ ' Always the fields of shortened # 0 0.5& • Lines that always shortened • Lines that shortened, then stretched • Lines that stretched, then shortened • Compare the result with that for simple shear • Lines that always stretched Shortened, then stretched • Lines that have a finite stretch of e>1 • Can you think of field ob!s ervations that help Always stretched to determine the kinematics of flow? Pure versus simple shear Practical determination in the field • Pure shear: • Shortened, then stretched lines distributed • Stretching/shortening can symmetrically around FSA be measured in the field, • E.g. from veins • Simple shear: • Plot all measurements in • Shortened, then stretched lines distributed stretch-direction plot asymmetrically around FSA • E.g. 0.8 in direction 040° • Equals 0.8 in direction 220° Practical determination in the field Practical determination in the field • Example of result for many • Example of result for many measurements measurements • Draw best-fit ellipse through data • e1 = 2.4 • e2 = 0.4 • Rf = 6 Practical determination in the field Only stretching • Example of result for many Only measurements shortening • Draw best-fit ellipse through data • The above Asaphus trilobite is deformed First shortening • e1 = 2.4 Then stretching • What is the finite strain ratio? • e2 = 0.4 • Rf = 6 • We need to know the original shape • So, what can you say about the kinematics of deformation? y x • Let us define the x-axis // to the axis of the trilobite Draw the Mohr • Current length is 129 mm circle • Assume original length 100 mm For strain • eaxis = 129/100 = 1.29 Assuming that the x- • We know that a normal Asaphus has a length-width • Current width // pleura is 59.6 mm axis did not rotate, ratio of 100 / 71.5 • Original width 71.5 mm what is the position gradient tensor? • The pleura are at 90° to the long axis of the trilobite • epleura = 59.6/71.5 = 0.83 • What is the finite strain ratio? • Pleura rotated !pleura = -29° (anti-clockwise) y f o r w a r x d reverse • Draw the Mohr circle for strain • Assuming our original size assumption was correct, what is the area change? • Assuming that the x-axis did not rotate, what is the position gradient tensor? • Now assume there was NO area change • What is the position gradient tensor (F)? • What was the original size of the trilobite? • Finally, draw the Mohr circle for reverse deformation • From the deformed state to the undeformed state • What is the position gradient for reverse strain (F-1).
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