Kinematics of the Greater Himalayan sequence, Dhaulagiri Himal ...

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$Fayek & Kyser 2000 uraninite fractionations.pdf - Queen's University$

329 330 331 332 333 Bouchez & Pêcher (1981) describe a broad zone of penetrative deformation, which they interpreted as extending more than 10 km south of (structurally below) the previously interpreted MCT (Figure 1c). They conclude that no large post-metamorphic strain affects their data and that the quartz c-axis fabrics measured reflect the kinematics of flow during the main deformation episode. 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 The present study builds upon the earlier work of Bouchez & Pêcher (1976), Bouchez (1978), and Bouchez & Pêcher (1981), extending it farther west to the Kali Gandaki valley (Figures 1c and 2). Oriented specimens of mylonitic paragneiss, orthoquartzite, orthogneiss, calc-silicate gneiss, and pelitic schist from across the Greater Himalayan sequence were collected for quartz c-axis orientation analyses. The sample locations are identified in Figure 7. Quartz c-axis orientations were measured optically, in thin sections cut parallel to lineation and perpendicular to foliation, using a universal stage mounted on a petrographic microscope. Orientation data from all samples are presented in equal area lower hemisphere projections whose plane of projection is perpendicular to the foliation and parallel to the lineation. The sense of shear motion along the foliation is indicated by half-arrows (Figure 8). Preferred crystallographic alignment of quartz was not observed in all samples. Samples from the middle and upper portion of the Greater Himalayan sequence (hollow circles in Figure 7) did not yield discernible c-axis patterns. This may reflect partitioning of strain into mineral phases other than quartz, such as calcite in calc-silicate gneiss, deformation temperatures that exceeded the preservation limits of quartz c-axis fabrics, and/or post-kinematic re-crystallization of quartz grains. In the following section, we describe the mineral assemblages, microstructures, and quartz c-axis fabrics from sampled specimens beginning with those from highest structural positions, progressing to those collected at lower structural positions. 16
352 4.1 Quartz petrofabric analysis results 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 Specimen 018 (Figure 8) is the structurally highest specimen that yielded a well-defined c-axis fabric (Figure 7). It is an amphibolite-grade paragneiss consisting of the assemblage of quartz, plagioclase, muscovite, biotite, garnet, ± kyanite. The foliation has an average strike of 266˚ and dips at 48˚to the NNW and is defined by aligned muscovite and biotite grains and elongate, plastically deformed quartz grains. A mineral shape lineation, defined by quartz, is visible on some foliation surfaces with an enigmatic orientation that plunges at 8˚ towards 286˚. This west-northwest plunge direction contrasts with a prevailing northeast-trending lineation from across the Greater Himalayan sequence. It is possible that sample 018 represents a local perturbation in flow, or perhaps that the specimen was sampled from a block that was slumped and rotated. The quartz in sample 018 exhibits abundant pinning structures, subgrain extinction, and a moderate foam texture (indicating Regime 3 recrystallization conditions of Hirth & Tullis [1992]) and is interpreted to have recrystallized through both subgrain rotation and grain boundary migration. Quartz c-axes define a moderately well-developed, slightly asymmetric, type-II cross-girdle pattern characterized by predominant prism slip (Figure 8). The asymmetric distribution of c-axes suggests a top-to-the-west-northwest sense of shear. 368 369 370 371 372 373 374 Specimen 015 (Figure 8) was collected from the quartzite unit at the base of the pelitic gneiss (Figure 7) near the contact mapped as the MCT by Colchen et al. (1986). It is a mylonitic orthoquartzite with a macroscopic foliation defined by aligned biotite mica that strikes 300˚ and dips 42˚ to the NE. The lineation present in the specimen, which plunges 28˚ towards 344˚, is also defined by aligned biotite grains. Quartz in specimen 015 is dominantly recrystallized through grain boundary migration resulting in a well-developed foam texture (Figure 3f; Regime 3 of Hirth & Tullis 1992). Quartz c-axis orientations yield a well-defined, type-I cross-girdle 17
Page 1 and 2: 1 2 3 4 5 Kinematics of the Greater
Page 3 and 4: 39 1. Introduction 40 41 42 43 44 4
Page 5 and 6: 84 85 86 87 88 provide new insight
Page 7 and 8: 129 130 131 132 133 134 135 The Dha
Page 9 and 10: 173 3.3 Greater Himalayan sequence
Page 11 and 12: 218 219 220 221 222 8 cm apart. In
Page 13 and 14: 263 quartzite and, locally, more im
Page 15: 307 308 309 310 311 312 In this stu
Page 19 and 20: 398 399 400 401 402 403 404 405 406
Page 21 and 22: 443 444 445 446 447 448 449 maximum
Page 23 and 24: 488 489 490 491 492 the X-Z plane o
Page 25 and 26: 534 535 through the following equat
Page 27 and 28: 572 573 574 575 576 577 578 579 val
Page 29 and 30: 617 618 619 620 621 622 623 624 625
Page 31 and 32: 663 6.3 Flow kinematics 664 665 666
Page 33 and 34: 709 710 711 712 713 714 715 716 717
Page 35 and 36: 753 754 better understand the proce
Page 37 and 38: 765 766 References 767 768 769 770
Page 39 and 40: 830 831 832 833 834 835 836 837 838
Page 41 and 42: 900 901 902 903 904 905 906 907 908
Page 43 and 44: 970 971 972 973 974 975 976 977 978
Page 45 and 46: 1039 1040 1041 1042 1043 1044 1045
Page 47 and 48: 1107 1108 1109 1110 1111 1112 1113
Page 49 and 50: 1163 1164 1165 1166 1167 a leucogra
Page 51 and 52: 1205 1206 1207 1208 1209 plotted ag
Page 53 and 54: 1247 1248 1249 1250 1251 1252 1253
Page 55 and 56: 83˚ 30'E South Tibetan Detachment
Page 57 and 58: A Previous Studies (Central Nepal)
Page 59 and 60: SW NE NW SE A B W E W E C D
Page 61 and 62: 018 015 grain shape foliation 038 2
Page 63 and 64: SW A S Qtz C Qtz Ep Am Am Am Ms 200
Page 65 and 66: 10 percent pure shear 80 70 60 50 4
Page 67:
(A) percent pure shear 60 50 40 30
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