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

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

421 422 423 slip, with a top-to-the-southwest asymmetry. The c-axes also appear to define a weak small circle about the pole to foliation. This may reflect a slight flattening strain (Schmid & Casey, 1986) possibly due to overthrusting of material above sample 078. 424 4.2 Temperature of deformation 425 426 427 428 429 430 431 432 433 434 435 436 437 438 Assuming that critical resolved shear stress values for operative glide systems are primarily controlled by deformation temperature, rather than strain rate or degree of hydrolytic weakening, the opening angle of cross-girdled quartz c-axis fabrics can be related to the temperature at which the fabrics were developed (Tullis et al. 1973; Lister & Hobbs 1980; Lister & Dornsiepen 1982; Kruhl 1998; Law et al. 1992; Law et al. 2004). During plastic deformation and dynamic crystallization, increased temperature results in an increased opening angle of quartz c-axis fabrics. The relationship between the opening angle of quartz c-axis fabrics and deformation temperatures is linear through greenschist to upper amphibolite facies conditions (Kruhl 1998; Law et al. 2004). However, beyond ~700˚C empirical data indicate that opening angles increase faster than temperature increases (Law et al. 2004). This relationship may reflect the activation of prism dominated slip. Kruhl (1998) estimated that temperatures derived from opening angles of quartz c-axis fabrics were subject to an uncertainty of ±50˚C. This uncertainty accounts for the range in strain rates and effects of hydrolytic weakening likely to be encountered during deformation (Law et al. 2004). 439 440 441 442 Quartz c-axis fabrics from specimens collected across the lower portion of Greater Himalayan sequence in the Kali Gandaki valley have little systematic variance in opening angle with respect to structural position from its base (Figure 9a). However, within the migmatic portion of the Greater Himalayan sequence the opening angle of c-axis fabrics increases to a 20
443 444 445 446 447 448 449 maximum observed angle of 87˚ (Figures 8 and 9b), which corresponds to a temperature of 670±50˚C (018; Figure 9b) using the geothermometer of Kruhl (1998) as modified by Law et al. (2004). Specimens sampled from structurally higher positions yielded random quartz c-axis fabrics, which precludes deformation temperature estimates in the upper-middle to-top portion of the extruded Himalayan infrastructure. Quartz c-axis fabric opening angles for quartzite specimens taken from the lower portion of the Greater Himalayan sequence indicate deformation temperatures of approximately 500˚C (049, 053, 070B, 078; Figure 9b). 450 451 452 453 454 455 456 457 458 Published temperature estimates for Neohimalayan equilibrium metamorphic assemblages in the Kali Gandaki valley, interpreted to be coincident with the southward extrusion of the mid-crust into the Himalaya (Vannay & Hodges 1996), can be compared with quartz c-axis derived temperatures from similar structural positions to determine if quartz c-axis fabrics were developed under comparable thermal conditions (Figure 9a). The temperatures estimated by each method are similar, thus the quartz c-axis fabrics are interpreted to have developed during the same Neohimalayan event as the metamorphic assemblage. The quartz c- axis patterns, therefore, reflect strain in the lower half of the Greater Himalayan sequence during Neohimalayan mid-crustal extrusion and not some other deformation event. 459 460 461 462 463 464 465 Combining the results of quartz c-axis and metamorphic assemblage geothermometry results in a more detailed temperature profile across the Greater Himalayan sequence in the Kali Gandaki valley, one which appears to reflect an inverse thermal field gradient (Figure 9a). A similar inverse thermal gradient across the exhumed Himalayan metamorphic core has been observed in other transects along the orogen (e.g., Ganser 1964; Hubbard 1989). Perturbations in the temperature profile across the Greater Himalayan sequence rocks exposed in the Kali Gandaki valley may be indicative of post-metamorphic thrusting (e.g., Kalopani shear zone, 21
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 and 16: 307 308 309 310 311 312 In this stu
Page 17 and 18: 352 4.1 Quartz petrofabric analysis
Page 19: 398 399 400 401 402 403 404 405 406
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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