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Chapter 10 Note to instructors Problems 10.5 and 10.6 could be posed in terms of overall strain instead of stress if the instructor desires. Answers for both approaches are given below. Answers Problem 10.1 See solution on p. 42. Problem 10.2 Orientations of principal stress axes will vary slightly among students. See solutions on p. 42: s1 ¼ 71� toward 212� s2 ¼ 18� toward 056� s3 ¼ 7� toward 323�: Problem 10.3 Orientations of principal stress axes will vary slightly among students. See solutions on p. 42: s1 ¼ 7� toward 118� s2 ¼ 10� toward 209� s3 ¼ 78� toward 353�. Problem 10.4 Strain: Regional east–west shortening after the Mississippian. Regional east–west extension followed shortening. Orientation of faults in part governed by bedding surfaces during shortening. Stress: Early deformation: s1 ¼ east---west, s2 ¼ north---south, s3 ¼ vertical: Later deformation: s1 ¼ vertical, s2 ¼ north---south, s3 ¼ east---west: Problem 10.5 Note that events 2 and 3 below are permissively the same event; it depends on whether granodiorite is considered to be folded. Strain: North–south shortening in southeast block between Mississippian and Cretaceous (F1 folds). East–west shortening in southeast block between Mississippian and Cretaceous but after north–south shortening (Cretaceous granodiorite could be folded) (F2 folds). East–west shortening through map area, post-Eocene, pre-Miocene (F3 folds). East–west extension, post-Miocene (normal faults). ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 40 9.1.2007 3:37pm Compositor Name: PDjeapradaban Stress: D1: s1 ¼ north---south, s2 ¼ east---west, s3 ¼ vertical: D2: s1 ¼ east---west, s2 ¼ north---south, s3 ¼ vertical: D3: s1 ¼ east---west, s2 ¼ north---south, s3 ¼ vertical: D4: s1 ¼ vertical, s2 ¼ north---south, s3 ¼ east---west: Problem 10.6 (1) s2 ¼ north---south, s3 ¼ east---west (s1 ¼ vertical): (2) s2 ¼ north---south, s3 ¼ east---west (s1 ¼ vertical): (3) s1 ¼ north---south, s3 ¼ east---west (s2 ¼ vertical): (4) The paradox is that s1 and s2 both have the same orientation. Stress ellipsoid orientations can be derived from answers 2 and 3 above. If the magnitudes of s1 and s2 are very close to one another, minor perturbations of the stress field could cause these stress axes to switch. (Note: Duebendorfer doesn’t believe this. He interprets the strike slip faults to be extensional transfer faults.) Problem 10.7 (1) See solutions on p. 43 (a and b). (2) See solution on p. 43 (c). (3) Clockwise rotation of s3. (4) Vertical principal stress could increase due to loading associated with volumin- ous magmatism. Vertical principal stress could decrease due to crustal thinning associated with normal faulting. Problem 10.8 (1) See solution on p. 44. (2) Not reproducible for manual. (3) Not reproducible for manual. (4) Key points: (a) Development of structures proceeds sequentially from south to north. (b) s1 oriented due north–south and s3 oriented due east–west early in collisional history. (c) About a 25–308 clockwise rotation in horizontal stress directions later in history as Southeast Asia is ‘‘extruded’’ from the main part of Asia. (d) In the far north s1 and s2 switch orientations such that s1 is vertical. Problem 10.9 See solution on p. 44. Problem 10.10 See solution on p. 45. ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 41 9.1.2007 3:37pm Compositor Name: PDjeapradaban 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Structural Analysis and Synthesis ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ 41 Solutions Problem 10.1 Problem 10.2 Problem 10.3 ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 42 9.1.2007 3:37pm Compositor Name: PDjeapradaban 42 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Problem 10.7 Explanation for conversion of a stike-slipfault from sinistral to dextral during one tectonic episode: _______________ Clockwise rotation of s3______________________________________________________________________________________________________ Speculations about the geologic factors involved in the structural development of this region: _____________________ Vertical principal stress could increase due to loading associated with volcanism ______________________________________________________________________________________________________ Vertical principal stress could decrease due to crustal thinning due to normal faulting ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 43 9.1.2007 3:37pm Compositor Name: PDjeapradaban 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Structural Analysis and Synthesis --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 43 Problem 10.8 Orientations of the two horizontal principal stresses will vary slightly because faults are not perfectly straight. It is probably best to tell the students to use a best-fit line to approximate the overall orientation of a given fault. For the Himalayan Frontal thrust, s3 is probably vertical; however, you may wish to accept a WNWoriented s3 instead of the solution shown here as the models of Tapponier et al. (1982) show approximately E-W extension. Problem 10.9 ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 44 9.1.2007 3:37pm Compositor Name: PDjeapradaban 44 ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Structural Analysis and Synthesis ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Problem 10.10 Yes, they are compatible (NW-SE shortening axes). The extension axes are different, but they are not incompatible (two directions of extension). ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 45 9.1.2007 3:38pm Compositor Name: PDjeapradaban ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Structural Analysis and Synthesis ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 45 Chapter 11 Note to instructors Reports will vary. Listed below, from oldest to youngest, are the main geologicevents embodied in the Bree Creek Quadrangle. There are a few relationships that are ambiguous with respect to timing. These are intentional and are designed to force the student into presenting evidence for his or her conclusions. 1. Deposition of Paleozoic rocks. 2. Folding of Paleozoic rocks about east–west axes. 3. Folding of Paleozoic rocks about north–south axes (this event is arguably correlative with the folding of Eocene rocks, but we favor separating these events). 4. Intrusion of the Cretaceous granodiorite. 5. Uplift and erosion to expose granodiorite and folded Paleozoic rocks. 6. Deposition of Paleocene to Eocene rocks. 7. Folding of these rocks about north–south axes. This was probably contempor- aneous with thrusting along the Mirkwood fault. 8. Uplift and erosion. 9. Deposition of Miocene rocks. Either non-deposition or erosion of Tg unit prior to Tr deposition on northeast block. 10. Normal faulting on all faults (possibly contemporaneous with tilting below). 11. Regional northward tilting. 12. Erosion: complete removal (or original non-deposition) of Miocene rocks in vicinity of Gandalf’s Knob. 13. Deposition of Pliocene unit. 14. Tilting to north. ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 46 9.1.2007 3:38pm Compositor Name: PDjeapradaban Chapter 12 Answers and solutions Problem 12.1 Problem 12.2 Problem 12.3 Higher strain rates in the hinges of folds result in brittle deformation. The limbs of folds, experiencing slower strain rates, deform continuously without fracturing. This problem reinforces the concept that strain rate is a very important aspect of rhe- ology; rocks that fracture at high strain rates will fold under relatively lower strain rates. Problem 12.4 These problems are intended to encourage students and their lab instructor to creatively explore the rheological properties of various materials. ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 47 9.1.2007 3:38pm Compositor Name: PDjeapradaban Chapter 13 Answers Problem 13.1 sn ¼ 77:53 MPa; ss ¼ 35:95 MPa Problem 13.2 See solution on facing page Problem 13.3 sn ¼ 29 MPa; ss ¼ 12 MPa Problem 13.4 (1) See solution on p. 50. (2) m ¼ 0:53 (3) u ¼ 59� Problem 13.5 Maximum spacing is 56.45 m (see solution on p. 50) Problem 13.6 m ¼ 0:90 (since students will measure slightly different u angles, the value of m will vary widely; most students will report values of 0.8 to 1.2 for m) Problem 13.7 Pore pressure ¼ 38 MPa Problem 13.8 Pore pressure ¼ 2.5 MPa ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 48 9.1.2007 3:38pm Compositor Name: PDjeapradaban Solutions Problem 13.1 s1 ¼ 100 MPa; s3 ¼ 20 MPa; u ¼ 32� sn ¼ (s1 þ s3)=2 þ [(s1 � s3)=2] cos 2u ¼ (100 þ 20)=2 þ [(100 � 20)=2] cos 64� ¼ 77:53 MPa ss ¼ (s1 � s3)=2 sin 2u ¼ [(100 � 20)=2] sin 64� ¼ 35:95 MPa Problem 13.2 Involves substitution into above equations (Problem 13.1). Plane u sn ss 1 328 77.5 MPa 36 MPa 2 458 60 MPa 40 MPa 3 908 20 MPa 0 MPa 4 1358 60 MPa �40 MPa 5 1808 100 MPa 0 MPa ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 49 9.1.2007 3:38pm Compositor Name: PDjeapradaban 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Structural Analysis and Synthesis --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 49 Problem 13.3 Problem 13.4 Problem 13.5 At failure (from Problem 13.4): s1 ¼ 50MPa ¼ 5:1� 106 kg=m2 Weight of tuff per m3: (2:0 g=cm3)(106 cm3=m3)(kg=1000 g) ¼ 2000kg=m3 Weight supported by pillar: [(pr2)(20m)(2000 kg=m3)]=[(2:5m2)p] ¼ 5:1� 106 kg=m2 r2 ¼ [p(2:5m2)(5:1� 106 kg=m2)]=[p(20m)(2000 kg=m3)] r ¼ 28:23m d ¼ 2r ¼ 56:46m maximum spacing ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 50 9.1.2007 3:38pm Compositor Name: PDjeapradaban 50 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13.7 Problem 13.8 ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 51 9.1.2007 3:38pm Compositor Name: PDjeapradaban 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Structural Analysis and Synthesis ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 51 Chapter 14 Answers and solutions Problem 14.1 (1) 0.10 (2) c ¼ 45�; g ¼ tan c ¼ 1:0 Problem 14.2 Problem 14.3 ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 52 9.1.2007 3:38pm Compositor Name: PDjeapradaban Problem 14.4 Results will vary slightly due to errors inherent in measurement. (1) e ¼ [11 � 10]=10 ¼ [1:7500 � 3:500]=3:500 ¼ �0:50. (2) e ¼ [11 � 10]=10 ¼ [3:500 � 3:500 ]=3:500 ¼ 0. (3) 1þ e1 : 1þ e2 ¼ 2 : 1. (4) Diagonal dike on left falls into zone 2. Thus this segment of the dike has under- gone early shortening followed by elongation, but with net shortening. Diagonal dike on right falls into zone 1b. Thus this segment of the dike has undergone early shortening followed by elongation, with net elongation. See below. Problem 14.5 Problem 14.6 Constructions will vary. Overall ellipse shape should be similar to that shown. Ratio ¼ 2 : 1; orientation of maximum principal strain ¼ 2938. ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 53 9.1.2007 3:38pm Compositor Name: PDjeapradaban ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Structural Analysis and Synthesis ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 53 Problem 14.7 ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 54 9.1.2007 3:38pm Compositor Name: PDjeapradaban 54 ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ Structural Analysis and Synthesis ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Problem 14.8 Note measured values will differ slightly, but overall ratios should be close. Semi-major axis 10 9 7 7 11 9 7 x–=8.6 Semi-minor axis 7 7 5 4.5 8 7 5 x–=6.2 Semi-major axis 24 19 17 11.5 18 8.5 15 x–=16.1 Semi-minor strain ellipsoid plots in field of apparent constrictionaxis 8 6 5.5 4 6 3 5 x–=5.4 1 +e2 1.4 1 +e3 1.0 :1 +e1 1 +e3 : : : 3.0 1.0 ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 55 9.1.2007 3:38pm Compositor Name: PDjeapradaban ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Structural Analysis and Synthesis ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ 55 Chapter 15 Note to instructors This chapter is designed to introduce students to the principles and a few funda- mental techniques of cross-section balancing. We strongly urge instructors to aug- ment the exercises in this chapter with local examples of unbalanced cross sections. For obvious reasons, we felt that it would be inappropriate to include in this chapter examples of unbalanced cross sections from the literature. ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 56 9.1.2007 3:38pm Compositor Name: PDjeapradaban Answers and solutions Problem 15.1 Problem 15.2 Panel Hanging wall Footwall A Ramp Flat B Flat Flat C Flat Ramp D Flat Flat E Ramp Flat F Flat Flat G Flat Ramp H Flat Flat I Flat Flat J Ramp Flat K Flat Flat L Ramp Flat M Flat Flat N Flat Ramp O Flat Flat P Flat Ramp Q Flat Flat ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 57 9.1.2007 3:38pm Compositor Name: PDjeapradaban ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Structural Analysis and Synthesis ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 57 Problem 15.3 Note to instructor. The problems present in this cross section can be stated in several different ways; i.e., if there is no hanging wall flat corresponding to a certain footwall flat, the reverse is also true. You will have to use your judgment in determining whether or not the students understand the concepts embodied in this exercise. Line lengths are not equal. (Note: scale may have changed during final printing.) Top of Jurassic ¼ 210 mm Base of Jurassic ¼ 211 mm Top of Triassic ¼ 245 mm Base of Triassic ¼ 247 mm Top of Permian ¼ 185 mm Base of Permian ¼ 188 mm A – no hanging wall flat corresponding to this footwall flat. B – no hanging wall ramp corresponding to this footwall ramp. C – no hanging wall flat corresponding to this footwall flat. D – hanging wall flat at the base of the Triassic unit is far too long. E – no hanging wall ramp corresponding to this footwall ramp. F – no footwall ramp that cuts off both the Jurassic and Triassic. Bottom line: There are three footwall ramps but only two hanging wall ramps. This discrepancy leads to problems with different units cut off in hanging wall and footwall ramps, number of hanging wall vs. footwall flats, and length of footwall vs. hanging wall flats. Section does not meet the template constraint. Problem 15.4 ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 58 9.1.2007 3:38pm Compositor Name: PDjeapradaban 58 ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Structural Analysis and Synthesis -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Problem 15.5 Fault-bend fold. About 36% shortening. Problem 15.6 (1) Fault-propagation fold. (2) About 37%. (3) See map. Would hit unit Te at crest of anticline. ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 59 9.1.2007 3:38pm Compositor Name: PDjeapradaban ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Structural Analysis and Synthesis ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 59 Chapter 16 Note to instructors We encourage the instructor to augment the exercises in this chapter with personal hand specimens and thin sections of deformed rocks. Answers and solutions Problem 16.1 Quartz deformed by crystal plastic processes as evidenced by highly elongate grain shapes (ribbons). Feldspar deformed by microcracking as evidenced by conspicuous loss of cohesion. Temperature of deformation was probably between 300 and 4508C. Quartz grains are largely unrecrystallized and show only minor evidence for recovery. Problem 16.2 Pressure solution (diffusive mass transfer). IP ¼ interpenetrating grain boundaries SG ¼ straight grain boundaries ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 60 9.1.2007 3:38pm Compositor Name: PDjeapradaban Problem 16.3 SG ¼ subgrains NG ¼ new grains Problem 16.4 Ultramylonite. Porphyroclasts make up less than 10% of rock. If the protolith was a coarse-grained granite, one would expect to see feldspar porphyroclasts. The absence of feldspar porphyroclasts indicates that the feldspars have undergone grain-size reduction. Problem 16.5 Mylonite. Porphyroclasts make up between 10 and 50% of rock. ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 61 9.1.2007 3:39pm Compositor Name: PDjeapradaban ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Structural Analysis and Synthesis ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 61 Problem 16.6 (1) The feldspar exhibits a discrete microfracture and therefore was deformed by brittle deformation processes (cataclasis). The quartz forms highly elongate ribbons and therefore was deformed by intracrystalline plastic deformation processes (dislocation glide and probably limited dislocation climb). (2) Temperatures high enough for quartz to deform plastically but not high enough for feldspar to deform plastically. Deformation therefore occurred between approximately 300 and 4508. (3) Dextral shear sense. AF = antithetic fracture SP = sigma porphyroclast DP = possible delta porphyroblast (weak) Problem 16.7 (1) Delta porphyroclast. (2) Shear sense ¼ reverse-sense motion; southeast side up. Problem 16.8 Shear sense ¼ normal-sense motion; southwest side down. Problem 16.9 Pre-135 Ma southeast-over-northwest reverse movement (thrusting) on shear zone C. Southeast-side down normal movement on shear zone B between 135 and 65 Ma. Post-65 Ma right-slip faulting on shear zone A. ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 62 9.1.2007 3:39pm Compositor Name: PDjeapradaban 62 --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Structural Analysis and Synthesis -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Chapter 17 Note to instructors You may wish to devote two lab periods to this chapter. Answers and solutions Problem 17.1 Problem 17.2 Problem 17.3 (a) Reverse-slip movement (with minor sinistral component) along east-striking, 708N-dipping fault, or reverse-slip movement (with minor dextral component) along N628W-striking, 308SSW-dipping fault. (b) Dominantly right-slip movement along N108W-striking, 808W-dipping fault, or dominantly left-slip movement along N788E-striking, 858S-dipping fault. (c) Dextral-normal slip along N508E-striking, 508NW-dipping fault, or sinistral- normal slip along N738W-striking, 508SW-dipping fault. ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 63 9.1.2007 3:39pm Compositor Name: PDjeapradaban Problem 17.4 Plots will vary slightly from student to student, particularly where oblique move- ments are involved. Plots can be generated visually or plotted precisely on stereonets recalling that the pole to the auxiliary plane is the slip direction. Students should assume reasonable dips for boundaries; in the plots below, we assume 908 dip for transform boundaries and 458 dips for convergent and divergent boundary faults. Problem 17.5 Event A occurred along the N858W-striking transform. Right-lateral movement. Event B occurred along the N128W-striking, ENE-dipping subduction zone. Move- ment was reverse-slip with a dextral component. ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 64 9.1.2007 3:39pm Compositor Name: PDjeapradaban 64 ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Structural Analysis and Synthesis ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Problem 17.6 (a) I ¼ 36:0. (b) I ¼ �63:4. (c) I ¼ 83:9. (d) I ¼ �19:4. Problem 17.7 Rate ¼ distance/time ¼ 810 km/18 Ma ¼ 45 km/Ma or 45mm/yr for half-spreading rate. Full spreading rate ¼ 90 mm/yr. Problem 17.8 Problem 17.9 The ‘‘plate game’’: this problem was inspired by a similar exercise devised by the late Dr. Peter Coney of the University of Arizona more than 30 years ago. In the exercise, North America is fast forwarded by about 100 Ma. Western California and Baja have been translated northwest and have collided with southern Alaska; the Basin and Range and Colorado Plateau (along the Rio Grande rift) have separated from the rest of North America. Listed on the next page are calcu- lated latitudes for paleomagnetic data presented. Also shown are the preferred positions of ocean ridge segments and associated transforms based upon correlation Age Inclination Latitude N-S distance* Ratey 60 Ma 46 64.2 38 Ma 33 52.4 1312 59.6 mm/yr 18 Ma 25 43.0 1045 52.3 mm/yr * Equal to degrees change in latitude multiplied by 111 km/degree. y Calculated rates are minimum values because paleomagnetic determinations can only detect the north–south component of movement (changes in latitude). ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 65 9.1.2007 3:39pm Compositor Name: PDjeapradaban ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Structural Analysis and Synthesis ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 65 of magnetic anomalies. Specific geologic features are named. Students could be asked to determine the ‘‘exact’’ time of rifting and rates of movement of tectonic blocks. We welcome suggestions, additions, deletions, and corrections to this exercise. 1. See above 2. See above 3. Answers will vary. Check for consistency with map. For example, if Colorado Plateau is considered ‘‘fixed’’, the North American Midcontinent is moving relatively southeast, and the North American Cordillera is moving relatively northwest. 4. Information item only (no answer). 5. Basically, this map tracks the separation and northwestern motion of the block west of the San Andreas fault (western California and Baja California) from the rest of North America until its arrival at the Aleutian trench. Because this block is composed of buoyant continental material, it cannotbe subducted and therefore collides with and is accreted to Alaska. Basin and Range extension ultimately causes separation of the North American Cordillera from the Mid- continent and the Colorado Plateau. Lesser extension along the Rift Grande rift results in separation of the Colorado Plateau from the Midcontinent. Locality Radiometric date (Ma) Inclination Latitude (calculated) P1 120 51.4 32.1 P2 50 68.0 51.1 P3 15 55.5 36.0 P4 2 58.7 39.4 P5 15 55.5 36.0 P6 2 52.0 32.6 ROWLAND / Structural Analysis and Synthesis xxxxxxxxxx_4_10-17 Final Proof page 66 9.1.2007 3:39pm Compositor Name: PDjeapradaban 66 --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Structural Analysis and Synthesis -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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