Batholith emplacement at mid-crustal levels and its exhumation within an obliquely convergent margin

ELSEVIER

Tectonophysics 312 (1999) 57–78 www.elsevier.com/locate/tecto

Batholith emplacement at mid-crustal levels and its exhumation within an obliquely convergent margin Maria Luisa Crawford a,Ł , Keith A. Klepeis b , George Gehrels c , Clark Isachsen c b Division

a Department of Geology, Bryn Mawr College, Bryn Mawr, PA 19010, USA of Geology and Geophysics, Bldg. F05, University of Sydney, Sydney, NSW 2006, Australia c Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA

Received 6 October 1998; accepted 17 March 1999

Abstract Emplacement of the central part of the Coast Mountains batholith of northern coastal British Columbia occurred within a regime characterized by oblique convergence between the Farallon=Kula and North American plates. We use new structural, kinematic and U–Pb isotopic data to show that the locations, geometry, and mechanisms of pluton emplacement within this batholith were controlled by displacements within a network of normal faults and transtensional shear zones. These data also show that the most active period of pluton emplacement, from ¾67 to ¾51 Ma, coincided with a change in style of deformation within the batholith. Prior to ¾67 Ma plutons were emplaced within an arc dominated by regional-scale contractional shear zones. In contrast, emplacement of 67–51 Ma plutons occurred in an arc increasingly dominated by normal faults with arc-parallel to oblique displacement and by sinistral transtensional shear zones. We have identified and mapped the structure of three plutonic complexes composed of 67 to 51 Ma plutons: the Khyex sill complex, Arden Lake plutonic complex and Quottoon plutonic complex. Shear-zone-controlled emplacement of plutons within the batholith accounts for the widely different orientations and structural features that characterize plutons within these three complexes. During and after this latest Cretaceous–Paleogene period of intense plutonic activity and accompanying deformation, the deep roots of the batholith were rapidly unroofed by ductile normal faulting prior to 50 Ma.  1999 Elsevier Science B.V. All rights reserved. Keywords: pluton; emplacement; shearing; deformation; mid-crust; British Columbia

1. Introduction In this paper, we examine the structure, kinematics, and timing of latest Cretaceous to Eocene pluton emplacement in the central part of the Coast Mountains batholith of northern British Columbia near 54.5ºN latitude. This work and other studies Ł Corresponding

author. Tel.: C1 610 526 5111; Fax: C1 610 526 5086; E-mail: [email protected]

of batholiths help reveal the mechanisms of crustal growth and the thermomechanical behavior of continental crust during orogenesis and arc construction at convergent margins (e.g. Petford and Atherton, 1992; Paterson and Fowler, 1993; Tobisch et al., 1995; Grocott et al., 1994; Tikoff and Saint Blanquat, 1997; Hollister and Andronicos, 1997). Among the specific issues we investigate in this paper are: (1) the mechanisms of pluton emplacement at mid-crustal levels within a deforming batholith; (2) the relationships

0040-1951/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 9 9 ) 0 0 1 7 0 - 5

58

M.L. Crawford et al. / Tectonophysics 312 (1999) 57–78

the Khyex sill complex, and the Arden Lake plutonic complex (Fig. 2). Individual plutons within these complexes exhibit distinctive structural elements, superposed high-temperature (600–700ºC) fabrics, and pluton–host rock relationships that have enabled us to reconstruct the pre-, syn-, and post-emplacement history of the central part of the Coast Mountains batholith using structural, kinematic, and U–Pb isotopic data. Results show that the entire batholith experienced three pulses of plutonic activity: (1) 88–70 Ma, (2) 67–63, (3) 59–52 Ma. The data also suggest that emplacement of the plutons between 67 and 52 Ma coincided with major arc-parallel to oblique displacements on gently dipping ductile normal faults and within a network of sinistral transtensional shear zones at mid-crustal levels within the deforming arc. These displacements controlled the location and geometry of pluton emplacement within this period and are compatible with extensional deformation described by Chardon and Andronicos (1997) and Andronicos et al. (1998). The ductile normal faulting and its increasing intensity toward the end of batholith construction between 55 Ma and 50 Ma also led to the tectonic denudation and exhumation of the roots of the batholith. Initiation of ductile normal faulting in this region in the latest Cretaceous and its greater than 15 Ma duration until 50 Ma explains why the deep roots of the central part of the Coast Mountains batholith were exhumed whereas those farther south were not.

2. Tectonic history of the central Coast Mountains Fig. 1. Tectonic map of the central Coast Mountains showing relative locations of the western thrust belt, the Coast shear zone, and the Coast Mountains batholith. QFC D Queen Charlotte transform fault.

between deformation patterns and pluton structure; and (3) the mechanisms by which the mid-crustal roots of the batholith and its surroundings were exhumed during regional oblique convergence. In the study area (Fig. 1), we have identified three plutonic complexes of markedly different orientations that are bounded by a network of regionalscale, interconnected shear zones. From west to east these complexes are the Quottoon plutonic complex,

In Cretaceous to Eocene time, the central Coast Mountains (Fig. 1) experienced a period of westward-vergent ductile thrust faulting, syntectonic pluton emplacement, and greenschist to amphibolite facies metamorphism (e.g. Monger et al., 1982; Crawford et al., 1987; Gehrels and Saleeby, 1987; Berg et al., 1988; McClelland and Gehrels, 1990; Crawford and Crawford, 1991; Rubin and Saleeby, 1992). This activity accompanied the accretion of a series of tectonostratigraphic terranes, some of them fartraveled, onto the western margin of North America (Monger et al., 1982; Crawford et al., 1987; Rubin and Saleeby, 1992; Klepeis et al., 1998). By

M.L. Crawford et al. / Tectonophysics 312 (1999) 57–78 59

Fig. 2. Structural and lithological map of a portion of the Coast Mountains batholith. The map includes areas previously mapped by Douglas (1986) and by L.S. Hollister (pers. commun., 1984, 1994). Note Khyex sill complex, Arden Lake plutonic complex and Quottoon plutonic complex. Cross-sections A–A0 , B–B0 and C–C0 are shown in Fig. 3. Circled numbers show location sites of the samples selected for isotopic dating. WC D Work Channel, QI D Quottoon Inlet.

60

M.L. Crawford et al. / Tectonophysics 312 (1999) 57–78

the mid-Cretaceous this period of convergence and accretion created a major crustal welt between the Alexander–Wrangellia and Stikine terranes (Monger et al., 1982). The exhumed mid-crustal roots of this welt, now exposed as the mid-Cretaceous thrust belt on the western side of the orogen (Fig. 1) record pressures up to 8–9 kbar (Crawford et al., 1979; Cook et al., 1991; Cook and Crawford, 1994). By 85–80 Ma, high P–T rocks in this western belt had cooled to <250ºC (Crawford et al., 1979, 1987; Cook and Crawford, 1994); little igneous activity occurred in the western belt between ¾90 Ma and the Miocene (Crawford et al., 1987; Cook and Crawford, 1994). Between ¾75 Ma and 51 Ma the plutons we include in the central Coast Mountains batholith were emplaced along the eastern margin of the accreted terrane complex (Hutchison, 1982; Crawford and Hollister, 1982; Hollister and Andronicos, 1997; this study). The part of the Coast Mountains batholith in our study area, also referred to as the Central Gneiss Complex, comprises a 75–100-km-wide belt of tabular plutons and sheeted sill complexes emplaced into orthogneiss and paragneiss country rocks (Fig. 2). The emplacement of individual plutons coincided with the development of major plutonbounding shear zones (described below) both within and at the boundaries of the batholith. Rapid uplift (2 mm=yr) that resulted in decompression and cooling of the batholith and its host rocks occurred at ¾50 Ma (Hollister, 1982; Crawford and Hollister, 1982; Hollister and Crawford, 1990; Hollister, 1993). By the mid-Tertiary, the dextral Queen Charlotte transform fault (Fig. 1) and numerous extensional and transtensional sedimentary basins (Rohr and Dietrich, 1992; Rohr and Furlong, 1995) formed along this part of the margin of western North America.

3. Batholith overview The central Coast Mountains batholith in the study area forms a crustal block that is separated from the Cretaceous thrust belt on the west by the Coast shear zone and from low grade metamorphic rocks of the Stikine terrane on the east by arrays of ductile and brittle normal faults (Figs. 1 and 2). The Coast shear zone is steep to subvertical, 1.5–

4 km wide, and was formed during emplacement of the Coast Mountains batholith (McClelland et al., 1992; Klepeis et al., 1998). In the area of this study, the highest strain zones of the shear zone cut and deform the westernmost parts of the Coast Mountains batholith. The ductile faults on the eastern side of the central Coast Mountains batholith include the Shames River shear zone (Heah, 1991) and the Portland Canal shear zone, recently mapped by Evenchick and Crawford (Evenchick et al., 1999). Movement on these and on the younger steep brittle faults uplifted the central Coast Mountains batholith relative to rocks on the east. Our study focuses on the western portion of the central Coast Mountains batholith crustal block (Fig. 2). Within the study area we have identified three plutonic complexes cut and surrounded by a network of regional ductile shear zones. From west to east these are the Quottoon plutonic complex, the Khyex sill complex, and the Arden Lake plutonic complex (Fig. 2). We distinguish these complexes on the basis of orientation, location within the batholith, and the texture and compositions of plutons within each complex. The range of ages of the igneous rocks in each of the complexes is similar. The host rocks of the batholith are a layered series of metasedimentary and amphibolitic rocks and orthogneiss with a regional dip of 30º–45º to the north and northeast across the central parts of the batholith (Hutchison, 1982); this fabric is locally reoriented into a northtrending vertical orientation. The Quottoon plutonic complex is a moderately to steeply east-dipping, ¾7–10-km-wide tabular body of mafic diorite to tonalite with minor granodiorite that lies along the western edge of the Coast Mountains batholith in the study area (Figs. 2 and 3). In earlier publications the Quottoon plutonic complex was referred to as the Quottoon pluton (Hutchison, 1982; Crawford et al., 1987). Recent work by Thomas (1998) and the age data summarized below demonstrate that the Quottoon plutonic complex comprises a number of individual intrusions. This complex can be traced continuously along the strike of the orogen for at least 70 km from the Skeena River south of the study area to the southern shore of Portland Inlet to the north. North of Portland Inlet the Quottoon plutonic complex divides into several discrete sills and smaller plutons. The Coast shear

M.L. Crawford et al. / Tectonophysics 312 (1999) 57–78

61

Fig. 3. Cross-sections. See Fig. 2 for locations. (a) NNE–SSW section across central part and roof zone of the Khyex sill complex. (b) E–W section across the Coast shear zone, Quottoon pluton, Khyex sill complex and western part of the Arden Lake plutonic complex.

zone that produced a steep to subvertical foliation along the western margin of the complex cuts the western side of the pluton complex. The eastern contact is a ¾1 km wide intensely deformed zone characterized by dikes and sills of variable composition that display mutually crosscutting relations and migmatitic textures. The other two complexes, Khyex sill complex and the Arden Lake plutonic complex, contain intrusions with compositions that include gabbro, tonalite, leucotonalite, and granodiorite. They are identified and distinguished from each other and from the Quottoon plutonic complex mainly by their structural and textural character and by location. East of the Quottoon plutonic complex, between 54º200 N and 54º350 N, a series of gently NE-dipping sheeted sills separated by orthogneiss and metasedimentary screens collectively form the Khyex sill complex (Fig. 2) (L.S. Hollister, pers. commun., 1984 and 1994; Hollister et al., 1994). Bounding all sides of the Khyex sill complex is a network of interconnected ductile shear

zones. These shear zones separate the Khyex sill complex from the Arden Lake plutonic complex on the east, from adjacent host rocks to the north and from the Quottoon plutonic complex on the west. The Arden Lake plutonic complex consists of orthoand paragneiss with a steep to vertical north-striking foliation that hosts a series of tabular plutons up to 500 m thick.

4. Ages of plutons In the study area igneous rocks assigned to the central Coast Mountains batholith yield ages that range from 88:5 š 0:8 Ma to 51 š 1 Ma (Fig. 4). We have subdivided the plutons into three age groups: 88–70 Ma, 67–63 Ma and 59–51 Ma. Ages were obtained using the standard isotope dilution method to analyze U–Pb isotopes in zircons. Analytical techniques are described in more detail by Gehrels et al. (1991a); the data are listed in Table 1.

62

M.L. Crawford et al. / Tectonophysics 312 (1999) 57–78

Table 1 U–Pb isotope data Grain characteristics

Apparent ages (Ma)

size

#

wt. (µg)

Pbc (pg)

U (ppm)

206m=204

Site 1

ZBa ZBa ZB ZBa ZB SB SB SC SC

1 1 1 1 1 12 12 10 10

34 63 139 147 94 500 420 520 405

14 13 8 36 12 1205 215 720 515

303 666 218 490 276 52 77 26 24

557 2310 2720 1431 1513 147 127 129 142

Site 2

ZD ZA ZB ZB ZF ZA ZB ZB ZB ZB

20 1 8 6 40 1 6 6 6 12

139 24 241 160 166 20 132 155 180 221

31 39 21 82 14 21 31 31 23 17

614 1020 176 1232 490 340 927 1010 944 490

Site 3

ZAa ZAa ZAa ZAa ZAa ZA ZA ZA ZA ZA ZA ZA ZA ZA

1 1 1 1 1 1 1 1 1 1 1 1 1 1

58 33 106 31 111 7 75 125 63 21 10 84 91 110

17 14 10 12 43 7 8 5 8 8 10 11 6 12

Site 4

ZB ZD ZF ZB ZB ZB

5 10 20 1 1 1

158 140 65 28 21 18

Site 5

ZA ZC ZF

5 20 50

Site 6

ZA ZD ZD ZF

Site 7

ZB ZD ZD ZF

206=208

206*=238

207*=235

207*=206*

5.2 7.6 5.5 7.6 6.4 1.9 1.9 1.7 1.9

73.1 š 0.6 72.2 š 0.4 72.2 š 0.4 71.7 š 1.0 71.9 š 0.4 58.0 š 1.9 58.9 š 1.3 59.5 š 0.6 59.3 š 0.6

72.9 š 1.0 72.1 š 0.6 72.1 š 0.7 71.9 š 1.4 71.8 š 0.6 57.9 š 3.7 57.1 š 4.2 58.9 š 3.9 59.8 š 3.5

69 š 19 70 š 15 68 š 18 79 š 22 68 š 14 51 š 120 16 š 170 35 š 150 79 š 130

1750 360 1088 1490 3500 1790 3100 4140 455 4150

7.4 5.9 10.6 8.3 10.3 8.7 13.1 11.9 8.1 11.1

63.8 š 0.4 54.3 š 0.8 54.4 š 0.5 62.2 š 0.3 59.4 š 0.4 54.5 š 0.4 60.6 š 0.3 67.5 š 0.3 59.4 š 1.1 78.9 š 0.4

63.5 š 0.6 54.2 š 1.5 54.4 š 0.6 62.1 š 0.5 59.5 š 0.4 54.7 š 0.7 60.9 š 0.4 67.9 š 0.4 60.1 š 1.6 79.7 š 0.5

240 213 380 486 407 724 457 374 254 567 565 254 85 89

690 431 3140 1016 820 644 3460 7200 1635 1065 446 1490 1150 690

15.2 14.5 18.2 21.3 18.5 9.6 18.5 18.9 17.2 9.7 19.6 15.4 13.7 5.9

79.8 š 0.7 79.5 š 0.7 78.2 š 0.3 77.5 š 0.3 77.2 š 0.7 77.3 š 1.1 75.6 š 0.4 75.4 š 0.4 75.3 š 0.6 74.2 š 0.4 72.5 š 0.8 74.8 š 0.6 77.5 š 1.3 78.1 š 1.0

80.0 š 0.7 79.7 š 0.9 78.5 š 0.5 77.9 š 0.5 77.5 š 0.9 77.2 š 1.3 75.9 š 0.6 75.6 š 0.5 75.0 š 1.3 73.9 š 0.5 72.5 š 1.2 75.0 š 1.1 77.4 š 1.5 78.5 š 1.5

85 š 6 87 š 17 87 š 6 90 š 5 86 š 5 75 š 20 86 š 13 82 š 6 68 š 33 63 š 7 73 š 28 81 š 27 76 š 27 91.3 š 30

18 8 12 8 8 11

381 122 600 223 281 584

2135 1395 2840 503 505 630

11.4 11.9 13.0 7.3 7.6 6.8

64.5 š 0.3 66.5 š 0.6 64.5 š 0.4 64.6 š 1.5 64.7 š 1.6 64.1 š 0.9

64.7 š 0.7 66.9 š 0.8 64.6 š 0.6 64.6 š 1.9 65.0 š 1.8 63.9 š 1.5

72 š 20 86 š 20 69 š 15 67 š 44 76 š 43 55 š 45

85 114 80

22 72 24

768 2320 9333

1540 1880 1590

10.0 11.2 10.6

50.8 š 0.3 50.7 š 0.4 50.5 š 0.3

51.0 š 0.5 51.0 š 0.5 50.7 š 0.5

62 š 19 64 š 16 62 š 18

5 20 20 50

95 205 185 205

32 33 39 16

600 2453 4420 2507

33000 10250 14050 20800

29.4 33.1 32.9 33.0

66.6 š 1.1 67.1 š 0.4 66.8 š 0.6 67.2 š 0.7

66.8 š 1.1 67.4 š 0.5 67.0 š 0.6 67.5 š 0.7

75 š 6 74 š 9 76 š 7 77 š 6

10 20 20 50

250 185 195 210

22 19 22 82

692 1030 1050 1142

5760 6990 6520 2100

16.6 14.8 15.5 12.3

72.4 š 0.4 72.5 š 0.4 71.8 š 0.4 71.9 š 0.6

72.6 š 0.6 72.6 š 0.6 72.1 š 0.6 72.0 š 0.9

80 š 14 77 š 12 83 š 13 76 š 21

51 š 16 51 š 60 56 š 15 60 š 16 62 š 10 63 š 26 76 š 10 81 š 8 87 š 43 105 š 7

M.L. Crawford et al. / Tectonophysics 312 (1999) 57–78

63

Table 1 (continued) Grain characteristics

Apparent ages (Ma)

size

#

wt. (µg)

Pbc (pg)

U (ppm)

206m=204

SB SB

10 10

520 455

205 380

105 215

148 142

Site 8

ZD ZE ZB ZC SA SA

20 30 8 12 8 10

161 169 214 207 240 262

8 18 18 19 1000 575

480 550 442 912 195 88

Site 9

ZA ZB ZE ZG

6 10 20 50

195 136 78 125

28 21 29 67

Site 10

ZA ZB ZG ZD SA SA

6 10 30 20 10 10

169 166 88 174 230 205

Site 11

ZA ZB ZC ZAa ZAa ZAa ZAa ZAa

7 18 15 1 1 1 1 1

Toon Lake

ZA ZA ZA ZA ZAa ZAa ZAa ZAa ZA ZA ZA ZA ZA ZA ZA SA SA

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 7 9

206=208

206*=238

207*=235

207*=206*

1.4 2.1

48.9 š 0.7 50.0 š 0.5

50.1 š 2.9 50.9 š 2.5

107 š 120 93 š 130

8150 4450 3700 8750 42 39

10.6 10.1 11.4 12.4 0.5 0.5

88.5 š 0.4 88.4 š 0.4 88.4 š 0.5 88.4 š 0.4 51.4 š 2.2 51.5 š 2.6

88.7 š 0.6 88.7 š 0.7 88.7 š 0.6 88.7 š 0.5 49.1 š 14 53.9 š 16

94 š 11 95 š 14 98 š 12 98 š 10 60 š 660 158 š 660

1990 995 2741 4070

9600.0 10300 16400 6080

19.7 16.1 16.8 17.0

67 š 0.8 69.4 š 0.7 66.9 š 0.9 67.2 š 0.5

66.9 š 0.9 69.6 š 160.8 67.2 š 1.0 67.4 š 0.7

63 š 13 79 š 14 75 š 13 76 š 14

19 41 20 17 185 265

759 1198 618 1805 475 322

3700 2410 3300 9300 2890 1360

19.6 17.7 20.9 23.2 0.1 0.1

51.1 š 0.5 51.5 š 0.4 50.9 š 0.4 49.9 š 1.7 50.5 š 3.2 50.1 š 0.8

51.0 š 0.6 51.5 š 0.6 50.9 š 0.5 50.0 š 1.7 50.0 š 3.3 50.2 š 1.0

49 š 17 52 š 22 53 š 18 50 š 13 25 š 16 54 š 31

205 168 123 31 38 29 31 36

77 25 18 5 6 11 4 4

551 435 430 81 58 132 68 36

885 1735 1750 310 395 219 360 185

6.2 6.9 7.4 8.5 5.3 3.3 4.6 3.4

59.5 š 0.3 59.1 š 0.3 59.3 š 0.4 58.2 š 3.7 59.8 š 2.2 60.5 š 2.3 61.3 š 2.2 59.2 š 2.7

59.5 š 0.7 59.2 š 0.5 59.4 š 0.6 58.0 š 3.9 60.6 š 2.3 59.8 š 2.8 61.1 š 2.4 59.7 š 3.6

60 š 24 62 š 17 63 š 16 50 š 58 93 š 36 30 š 61 52 š 40 78 š 73

75 65 33 26 30 36 91 84 102 39 37 28 76 61 13 470 355

93 57 76 85 12 28 14 14 7 9 8 11 7 6 47 1505 335

2208 1298 1760 1981 1337 723 803 1862 242 620 398 1816 1561 1247 1355 25 19

1241 1038 640 560 2606 806 4740 8830 2200 2110 1340 366 1276 932 283 63 164

32.2 23.2 15.6 15.6 19.6 27.0 18.5 27.8 14.5 24.7 17.1 18.2 23.8 34.4 26.3 1.3 2.3

70.6 š 0.5 71.3 š 1.4 83.1 š 0.8 90.0 š 0.9 73.6 š 0.3 82.8 š 0.4 90.6 š 0.4 77.4 š 0.3 63.0 š 0.5 74.8 š 0.5 75.2 š 0.7 71.2 š 0.9 74.2 š 0.4 69.7 š 0.6 69.7 š 0.4 51.8 š 1.3 51.3 š 0.6

71.0 š 0.6 71.6 š 1.6 84.2 š 1.0 91.9 š 1.1 73.8 š 0.4 83.5 š 0.6 92.2 š 0.6 78.0 š 0.5 63.2 š 0.7 74.8 š 0.9 75.9 š 1.2 71.1 š 1.0 74.1 š 0.5 69.9 š 0.5 69.7 š 0.6 52.0 š 8.3 51.3 š 2.6

86 š 15 81 š 26 117 š 13 143 š 12 80 š 6 101 š 11 134 š 9 96 š 6 71 š 20 75 š 23 98 š 30 67 š 10 73 š 7 75 š 16 68 š 6 85 š 410 52 š 120

All uncertainties are at the 95% confidence level; * D radiogenic Pb; Z D zircon; S D sphene. Grain size: A D >175 µ, B D 145–175 µ, C D 125–145 µ, D D 100–125 µ, E D 80–100 µ, F D 63–80 µ, G D 45–63 µ; a D abraded; Pbc D total common Pb in picograms; 206m=204 is measured ratio, uncorrected for blank, spike, fractionation, or initial Pb; 206=208 is corrected for blank, spike, fractionation, and initial Pb. Pb and U concentrations have uncertainties of up to 25% due to uncertainty in grain weight. Constants used: ½235 D 9:8485 ð 10 10 ; ½238 D 1:55125 ð 10 10 ; 238=235 D 137.88. Pb blank generally ranged from 2 to 10 pg. U blank was <1 pg. Isotope ratios are corrected for fractionation of 0:14 š 0:10%=amu for Pb and 0:20 š 0:40 for UO2 . Initial Pb composition interpreted from Stacey and Kramers (1975), with uncertainties of 1.0 for 206=204, 0.3 for 207=207, and 2.0 for 206=208. All analyses conducted using conventional isotope dilution and thermal ionization mass spectrometry, as described by Gehrels et al. (1991a).

64

M.L. Crawford et al. / Tectonophysics 312 (1999) 57–78

Fig. 4. U–Pb concordia diagrams arranged by age and by location. Circled numbers in Fig. 2 show sample sites.

M.L. Crawford et al. / Tectonophysics 312 (1999) 57–78

The oldest plutons (Sites 1, 3, 7, 8, and Toon Lake, Fig. 2) have ages that range from ¾89 Ma to ¾72 Ma (Fig. 4). Three samples of tonalite orthogneiss interlayered with metasedimentary host rocks (Sites 1, 7 and 8) yield apparently concordant ages ranging from ¾89 Ma to 72 Ma. These metaigneous rocks older than 70 Ma share the oldest structures preserved in the batholith with their country rocks. This oldest fabric is a moderately north-dipping gneissic foliation where it is undeformed by younger phases of deformation. Also within this group of oldest plutons are units that yield complex U–Pb isotopic data. As shown in Fig. 4, an orthogneissic granodiorite sill at site 3 and a deformed granodiorite from Toon Lake pluton yield analyses that generally overlap concordia but range from ¾80 to ¾70 Ma. A few additional grains from the Toon Lake pluton are discordant with older dates, and one grain is apparently concordant at ¾63 Ma. The most likely explanation for this is that the dates result from crystallization near the young end of the range (¾72.5 Ma for the Site 3 sample and ¾70 or perhaps 63 Ma for the Toon Lake sample) with most, and perhaps all, grains containing inherited components of slightly older age. The presence of inherited components is supported by the fact that abraded grains tend to be older than unabraded grains. The crystallization age cannot be determined because the inherited components are sufficiently young that discordia lines are essentially parallel to concordia. Alternatively, the discordance in these two samples could result from nearly complete Pb loss at ¾70–80 Ma from grains of pre-80 Ma age. A similar mechanism has recently been documented in Late Proterozoic granulite facies metamorphic rocks of Madagascar (Tucker et al., 1998). 4.1. Khyex sill complex The Khyex sill complex lies in the central part of the study area (Figs. 2 and 3). One group of igneous rocks of this complex yields concordant ages between 67 and 63 Ma. These bodies are tabular and concordant to the foliation in the local country rock but lack solid-state fabrics except at their margins. Individual tabular sills are up to 2 km thick. Two bodies that cut pre-70 Ma gneissic bodies (Sites 3 and 7) were chosen to date this younger group:

65

a 64:5 š 1:5 Ma sill (Site 4) and a 67 š 1:0 Ma pegmatite dike from the eastern side of the complex (Site 6). 4.2. Arden Lake plutonic complex In the southeastern corner of the study area plutons up to 500 m thick form the Arden Lake plutonic complex. One of these, a deformed granodiorite, yields a concordant age of 67 š 1 Ma (Site 9). An undeformed vertical, north-striking tabular tonalite pluton (Site 11) yields an age of 59:3 š 1:0 Ma. 4.3. Quottoon plutonic complex The oldest phases of the Quottoon plutonic complex, as in the Arden Lake plutonic complex, fall in the 67–63 Ma age range (65 š 1 Ma, Klepeis et al., 1998; 65.6 Ma, J.A. Thomas, pers. commun., 1998). These lie along the eastern side of the Quottoon plutonic complex (Fig. 5) and range from gabbro through tonalite to granodiorite in composition. Other similar plutons that outcrop north of Portland Inlet (Fig. 5) also yield ages that range from 66 to 63 Ma (e.g., Klepeis et al., 1998). The younger rocks of the Quottoon plutonic complex make up the main phase of this complex. These consist of thick tabular mafic plutons that lack pervasive solid state deformation. They include a 55:5 š 1:5 Ma pluton from north of Portland Inlet (Klepeis et al., 1998), 55 Ma tonalite samples from Quottoon Inlet (Site 12, Fig. 2; J.A. Thomas, pers. commun., 1998) and a 58:6 š 0:8 Ma phase of the Quottoon plutonic complex sampled along the Skeena River, south of the study area (Gehrels et al., 1991b). One of a number of thin tabular bodies that lie between the Quottoon plutonic complex and the Khyex sill complex has an age of 54:3 š 2 Ma (Site 2), similar to the ages of the younger phase bodies of the Quottoon plutonic complex. 4.4. Widespread younger dikes and sills Throughout the area are thin granitic pegmatite and aplite dikes and sills less than 50 m thick with ages that cluster around 51 Ma (Site 5 and Site 10). One setting for these is along and within meter-scale shear zones that cut the Khyex sill complex and

66

M.L. Crawford et al. / Tectonophysics 312 (1999) 57–78

Fig. 5. Schematic map of the Quottoon pluton. Older (67–63 Ma) phases are shown with vertical line pattern, the younger (59–52 Ma) phases are horizontally lined. Equal-area nets summarize structural relations within the pluton, along its margins, and in the country rocks to the east. C D average lineation trend.

Arden Lake plutonic complex. Other bodies are E– W-striking granite dikes and sills with moderate to steep dips that cut all ductile fabrics. These dikes and sills and plutons of similar composition, age and crosscutting relations along Portland Canal to the north, appear to be very late to post-tectonic.

crosscutting relationships between rock fabrics, plutons and dikes within the complexes. We use these relationships to characterize and compare the overall styles of deformation that accompanied and postdated the emplacement of plutons during the three time intervals. 5.1. Oldest plutonic phase (88–70 Ma)

5. Relationships between deformation and pluton emplacement U–Pb isotopic data show that the plutonic complexes experienced three pulses of plutonic activity: (1) 88–70 Ma, (2) 67–63, (3) 59–52 Ma (Table 2). In this section, we discuss structural, textural and

Igneous rocks older than 70 Ma display structures that are identical in orientation, geometry and subsolidus texture to those in the amphibolitic and sedimentary country rocks. These structures include a moderately (30º–45º) N-dipping gneissic foliation (S1 ) and moderately N-plunging down-dip amphi-

M.L. Crawford et al. / Tectonophysics 312 (1999) 57–78

67

Table 2 Time–space relationships diagram

with Dcsz 3 of Klepeis et al. (1998). Correlated with Dcsz 2 of Klepeis et al. (1998).

a Correlated b

bole and biotite lineations. The oldest generation of plutonic rocks consistently display solid-state textures characterized by abundant evidence for dynamic recrystallization of quartz and by pressure shadows around ellipsoidal feldspar grains. Within the main S1 foliation C0 shear bands, oblique secondary foliations, and asymmetric tails on feldspar porphyroclasts consistently show top-up-to-the-south and -southwest, thrust-type displacements (Fig. 6). These fabrics are consistent with similar thrust-style fabrics described by Hollister and Andronicos (1997) elsewhere in the batholith. However, in contrast with fabrics present in most of the younger igneous rocks of the batholith, these pre-70 Ma rocks display no evidence of melt-present or migmatitic textures or of flow alignment of feldspar and amphibole grains. The complete recrystallization of these rocks has obliterated all evidence of fabrics formed during the emplacement of these plutons.

5.2. Middle plutonic phase (67–63 Ma) Structural and crosscutting relationships between host rocks and the 67–63 Ma plutons are best preserved in the central parts of the Khyex sill complex. Host rocks within this complex are metasedimentary and amphibolitic gneiss and older orthogneiss sills that contain a moderately N- and NE-dipping (28º–45º) gneissic foliation and mostly N- to NWplunging mineral lineations. Viewed on surfaces that parallel these lineations and are perpendicular to the gneissic foliation, kinematic indicators, including shear bands, S–C fabric, and asymmetric recrystallized tails on feldspar porphyroblasts, show evidence of top-up-to-the-south and -southwest kinematics identical to those of the oldest D1 regional fabric outside the sill complex. At the regional scale, the margins of the main 7–8 km thick body of coarse-grained tonalite, granodior-

68

M.L. Crawford et al. / Tectonophysics 312 (1999) 57–78

Fig. 6. Asymmetric delta-type tail on garnet porphyroblast within the L1 –S1 fabric in metasedimentary gneiss located north of the Khyex sill complex. Tail is composed of recrystallized quartz and feldspar aggregates. Asymmetry indicates top-up-to-the-south kinematics. South is to the right.

ite, and minor gabbro sills that comprise the Khyex sill complex parallel the country rock foliation. However, at the mesoscale these sills locally crosscut the S1 foliation and the margins of the older (pre-70 Ma) plutons. The 67–63 Ma sills are dominated by a gently and moderately N- and NE-dipping marginparallel foliation that, in sill interiors, displays prefull-crystallization-type fabrics (Hutton, 1988) with little to no evidence of subsolidus dynamic recrystallization. N- and NNW-plunging mineral lineations are defined by the alignment of euhedral feldspar, amphibole and biotite grains. Melt-filled shear bands in the sills record down-to-the-north normal displacements. At the edges of these sills the fabric displays subsolidus textures defined by the preferred shape orientation of flattened quartz–feldspar aggregates, biotite and amphibole grains. The D1 fabric in the host rock screens is variably overprinted and deformed by the younger D2 fabric. In places the only evidence for the older fabric are pods of rock that preserve S1 enveloped by the younger S2 foliation. Where subsolidus textures are evident the D2 fabric can be distinguished from the older D1 fabric because C0 shear bands, S–C fabrics, and asym-

metric tails on feldspar porphyroclasts of the D2 fabric consistently record top-down-to-the-north and -northwest normal displacements which is opposite to the displacement sense in the older fabric. In one area of the Khyex sill complex, tonalitic host rocks displaying magmatic textures are affected by sinistral transtensional shear zones that formed around minor mafic dikes. Many of these dikes are stretched parallel to well-aligned feldspar and amphibole grains (L2 ). Some mafic boudins form asymmetric fish that are deformed by melt-filled shear bands indicating top-to-north, oblique-sinistral displacements. These dikes also display mutually crosscutting relationships with respect to L 2 in the tonalite host rock suggesting that the deformation accompanied pluton emplacement. The oldest intrusive phases of the Quottoon plutonic complex also have ages in this range. These plutons, shown in Fig. 5, lie along the eastern side of the Quottoon plutonic complex. In the central part of the area these older rocks of the Quottoon plutonic complex are indistinguishable from bodies of the Khyex sill complex of the same age and are not distinguished from them in Figs. 2 and 3. The plutons

M.L. Crawford et al. / Tectonophysics 312 (1999) 57–78

are tabular and several kilometers thick; at their margins they break up into sills separated by screens of country rocks. These rocks comprise a wide range of compositions including ultramafic cumulates, quartz diorite, minor tonalite, and granodiorite. They contain melt-present textures, a penetrative, NE-dipping foliation (S2 ) defined by the alignment of flattened enclaves, quartz–feldspar aggregates, and amphibole and biotite grains, and NW-plunging biotite and amphibole mineral lineations. Weakly deformed synplutonic mafic dikes cut abundant mafic enclaves. Although not as well preserved due to later deformation described below, the fabrics within 67– 63 Ma tabular plutons of the Arden Lake plutonic complex also show evidence for melt-present deformation and orientations identical to the D2 fabrics in the country rocks. Within the country rocks the dominant foliation (S2 ) is axial planar to intrafolial rootless isoclinal folds and formed in the presence of the migmatite leucosome melt phase. A feature common to igneous bodies of the 67– 63 Ma age group from all three complexes is the presence of mafic synplutonic dikes generally less than several meters thick. These synplutonic dikes parallel the S2 foliation and exhibit the same tabular shapes as the sills into which they were emplaced. 5.3. Youngest plutonic phase (59–55 Ma) Tonalite and leucotonalite rocks with ages between 59 and 55 Ma are undeformed except along the western margin of the Quottoon plutonic complex which is cut by the youngest phase of deformation within the Coast shear zone (Table 2) (Ingram and Hutton, 1994; Klepeis et al., 1998). Prior to emplacement of this youngest group of rocks all older fabrics and igneous rocks were folded by tens of meters to kilometer-scale upright to recumbent folds (F3 ). Older plutons within the 59–55 Ma age range include the 59–58 Ma plutons in the Arden Lake plutonic complex and in the Quottoon plutonic complex along the Skeena River. In the Arden Lake plutonic complex these tabular plutons are parallel to a subvertical, north-striking foliation within a wide shear zone of similar orientation in the country rocks. Unlike the older 67–63 Ma plutons in the Arden Lake plutonic complex, the central portions

69

of these igneous bodies are not lineated or foliated. Some preserve a random magmatic fabric with meltpresent shears that are indistinct and display variable orientations. The largest pluton in the study area is the young ¾55 Ma phase of the Quottoon plutonic complex. This phase is homogeneous, coarse-grained quartz diorite and tonalite distinguishable from the 67–63 Ma Quottoon plutonic complex plutons by grain size, uniform composition and dominantly igneous textures. Thomas (1998) has shown that these younger Quottoon pluton complex rocks are also chemically distinguishable from the 67–63 Ma bodies. The igneous textures are defined by large equant amphibole, feldspar and quartz grains showing good to poor alignment, an absence of pressure shadows around porphyroclasts and no evidence of dynamic recrystallization. In places the alignment of these grains define a moderate to steeply NE-dipping weak magmatic foliation. A penetrative steeply plunging down-dip mineral lineation is defined by aligned tabular feldspar and amphibole grains in rocks near the center of the pluton. Equal-area plots in Fig. 5 illustrate the orientations of these structures. Pegmatite and aplite dikes are well developed in the interiors of some parts of this 55 Ma part of the Quottoon plutonic complex. These are predominantly E–Wstriking and dip at steep or low angles to the north. The orientation of the dikes suggests N–S to vertical extensional directions within the solidifying pluton. 5.4. Shear zones and their relationship to plutons The interval accompanying and following emplacement of the 67–63 Ma plutons and prior to emplacement of the 59–55 Ma plutons saw the formation of a network of interconnected regional-scale ductile shear zones that separate the three igneous complexes from each other and from adjacent blocks composed primarily of metamorphic country rocks (Fig. 2). These shear zones crosscut or reorient all penetrative fabrics within the study area. They display a wide variety of orientations including subvertical on the western side of the Arden Lake plutonic complex (Fig. 3b), gently and moderately NE-dipping on the northern side of the Khyex sill complex (Fig. 3a), and moderately to steeply E- to NE-dipping between the Khyex sill complex and the Quottoon

70

M.L. Crawford et al. / Tectonophysics 312 (1999) 57–78

Fig. 7. (a) L4 –S4 fabric within a shear zone that formed at the roof zone of the Khyex sill complex. Note tight folds of felsic veins within the shear zone. See Fig. 2 for orientation of structures. North is to the left. (b) Asymmetric boudinage and synthetic shear band within the L4 –S4 fabric at the roof of the Khyex sill complex. Note truncation of older fabric at the upper right of photograph. Asymmetry indicates top-down-to-the-north kinematics. North is to the left.

plutonic complex. The largest shear zones display widths ranging from less than 1 km to greater than 5 km. Within the study area these kilometer-scale

shear zones everywhere contain a solid-state foliation and N- to NW-plunging sillimanite, biotite, and amphibole mineral lineations (Fig. 7). The fabric in

M.L. Crawford et al. / Tectonophysics 312 (1999) 57–78

the Khyex sill and Arden Lake shear zones records an oblique-sinistral, north-side-down sense of displacement inferred from the orientation of gently Nand NNW-plunging to horizontal mineral lineations, the presence of numerous sinistral shear bands and asymmetric tails on mafic clasts and feldspar porphyroblasts. The shear zone located between the Khyex sill and the Quottoon pluton records both sinistral and, locally, minor dextral displacement. One of the widest of these shear zones is the vertical sinistral shear zone at the western side of the Arden Lake plutonic complex that hosts the steeply dipping undeformed younger (59 Ma) tabular bodies. This shear zone cuts and deforms 67–63 Ma granodiorite sills (e.g. Site 9, Figs. 2 and 4). The deformed granodiorite and the host rocks share a moderately N-plunging sillimanite, biotite and amphibole lineation and sinistral kinematic indicators including shear bands and asymmetric boudins in the country rocks and asymmetric, recrystallized tails on feldspar augen in the granodiorite. Several interconnected shear zones lie along most of the length of the eastern side of the ¾55 Ma pluton of the Quottoon plutonic complex. South of Quottoon Inlet and west of the Khyex sill complex (Fig. 2) the ¾1 km wide contact zone displays features that indicate deformation at high temperatures and in the presence of melt. Melt-filled shear bands and mafic clasts in a migmatitic matrix display asymmetric tails composed of felsic melt. Well developed moderately to steeply north-dipping sinistral and minor dextral shears contain gently plunging northwest-trending mineral lineations. These ductile fabrics crosscut fabrics in the country rocks and the igneous bodies of the Khyex sill complex to the east. We have seen no evidence of large-scale dextral shearing along the eastern Quottoon margin similar to that reported from south of this area by Hollister and Andronicos (1997). In this area older (67–63 Ma) and younger (59–55 Ma) Quottoon plutonic complex intrusions cannot always be distinguished due to the absence of unambiguous crosscutting relationships. However, it is always the case that where sequential emplacement can be demonstrated, the older plutonic rocks are deformed and the younger plutonic rocks show little evidence of deformation except at sill and dike margins. We interpret these foliations at sill and dike margins as magmatic be-

71

cause the hornblende prisms and plagioclase tablets show no evidence of subsolidus deformation. This leads us to infer that the deformation preceded crystallization of the younger intrusions. In the northwestern part of the study area (Fig. 2), the eastern contact of the Quottoon plutonic complex is marked by a 1 km wide zone of leucocratic orthogneiss that becomes more migmatitic toward the contact. In the contact zone there is a well developed foliation within a moderately to steeply ENE-dipping shear zone with an oblique-sinistral, north-side-down sense of motion that contains a strong, moderately (30–60º) plunging, north-trending mineral lineation (Fig. 5). In many places the rocks are L-tectonites. To the southeast this shear zone merges with and exhibits the same sense of shear as the gently northdipping shear zones that form the northern margin of the Khyex sill complex (Fig. 2). Melt-present deformation is widespread in this pluton contact zone. More than a kilometer from the pluton contact to the east, country rocks are less migmatitic and preserve older fabrics. These older fabrics are obliterated by the shear zone closer to the pluton contact. In the contact zone aplite and pegmatite dikes occur in the country rock. These dikes are folded into open folds, have a foliation parallel to the country rock foliation, especially in fold hinges, and are strongly boudinaged in two directions, both down-dip and approximately parallel to the stretching lineation. Where crosscutting felsic dikes record a sequence of emplacement, the older dikes show tight to isoclinal folds in contrast with more open folds in the younger dikes. Just south of this area along the shores of Quottoon Inlet a gently north-dipping foliation in the country rocks and a well developed gently northplunging mineral and stretching lineation are folded and transposed parallel to the margin of the Quottoon plutonic complex. The presence of the dikes and migmatite only in the zone adjacent to the pluton margin suggests they were generated by heating of the country rocks during pluton emplacement. In addition to the major shear zones described above, numerous smaller shear zones ranging in size from 1 m wide and several meters long to 500 m wide and 10 km long occur within the Khyex sill complex and in the rocks to the east. These minor shear zones display north-side-down sinistral and dextral kinematics (Fig. 8) and everywhere are

72

M.L. Crawford et al. / Tectonophysics 312 (1999) 57–78

M.L. Crawford et al. / Tectonophysics 312 (1999) 57–78

the youngest ductile structures. A systematic study along several transects resulted in the observation that all shear zones with dips steeper than 62º contain pegmatite or aplite dikes at their centers. The igneous dike rocks show a foliation at their edges parallel to the ductile fabric in the shear zones but retain igneous textures at their centers. Some of the larger shear zones (several meters wide) display melt-filled shear bands and migmatitic textures suggesting that deformation and intrusive activity occurred together. Along its western margin the Quottoon plutonic complex is overprinted by the west-side-up vertical youngest phase of deformation in the Coast shear zone (Table 2; D4CSZ phase of deformation of Klepeis et al., 1998). The shear fabric is most well developed in the country rocks; it decreases eastward into the pluton. Along and north of Quottoon Inlet steeply dipping to vertical shear zones that lie parallel to and show similar west-side-up displacement to this youngest Coast shear zone fabric are locally found within the pluton as well as in the gneiss at its eastern margin. Since the shearing is superimposed on the rocks of the western Quottoon plutonic complex, this deformation must post-date pluton emplacement. As shown in Fig. 5 this phase of the Coast shear zone displays a steeply dipping fabric and a near vertical mineral lineation. The orientation of these structures is distinctly different from the gentle to moderate northeasterly dip of foliations and the N- to NW-plunging lineations that characterize the shear zones in the country rocks just east of the Quottoon plutonic complex. North of the study area this youngest phase of the Coast shear zone was dated at ¾55 Ma based on the age of trondhjemite pegmatite bodies cut by the shear zone (Klepeis et al., 1998). Where the young phase of the Quottoon plutonic complex is wide and well developed, this late phase of the Coast shear zone is also wide (¾5 km) and contains numerous trondhjemite pegmatite bodies tens of centimeters to tens of meters wide. These appear to have been emplaced during defor-

73

mation and may have originated by partial melting of amphibolitic gneiss as a result of heat from the young Quottoon pluton phase. In contrast, where the Quottoon plutonic complex narrows and pinches out to the north, the late phase of the Coast shear zone narrows to a few hundred meters. This last stage of Coast shear zone deformation outlasted the emplacement of all phases of the Quottoon plutonic complex and continued as that pluton cooled. It is not clear whether the young Coast shear zone truncated and displaced the western margin of the Quottoon plutonic complex or whether it simply obliterated the contact relations.

6. Interpretation of deformation-controlled pluton emplacement To illustrate how the central Coast Mountains batholith evolved in different locations through time, we have displayed space–time relationships that incorporate our geochronologic, structural and kinematic data in tabular form (Table 2) and in diagrammatic form (Fig. 9). These summarize our evidence that arc-parallel or slightly oblique displacements played an increasingly important role in the deforming batholith during the transition from the latest Cretaceous to the mid-Tertiary. Our data suggest the following scenario: by 67–65 Ma N- to NW-directed normal and sinistral displacements initiated at the roots of the evolving batholith, following the 88–70 Ma phase of pluton emplacement and D1 ductile deformation (Table 2). The main phase of pluton emplacement, represented by 67–63 Ma plutons, occurred simultaneously with the onset of these D2 arc-parallel strike-slip and normal displacements inside the batholith (Fig. 9A). Kilometer-scale upright and recumbent folds with northplunging hinges deformed these plutons during D3 . After 60 Ma, steeply dipping tabular tonalite sills, including much of the Quottoon plutonic complex,

Fig. 8. (a) Horizontal view of the sinistral component of a sinistral transtensional shear zone from within the Khyex sill complex. Shear zone affects L2 –S2 fabric within a 67–63 Ma sill. North is to the left. (b) Vertical, cross-sectional view of the north-side-down normal component of motion within a sinistral transtensional shear zone from within the Khyex sill complex. Shear zone affects L2 –S2 fabric within a 67–63 Ma sill. North is to the right. These observations are compatible with the models of extensional unroofing and pluton emplacement proposed by Hollister and Andronicos (1997), Chardon and Andronicos (1997) and Andronicos et al. (1998).

74

M.L. Crawford et al. / Tectonophysics 312 (1999) 57–78

Fig. 9. Two-phase model of deformation accompanying emplacement and denudation of Coast Mountains batholith (figure adapted from Klepeis and Crawford, in press). Bold surfaces are major pluton-bounding shear zones; dashed lines show displacement directions; shaded areas are plutons; bold stripe is the young 59–55 Ma phase of the Quottoon pluton. (A) D2 and D3 during and after 67–63 Ma sill emplacement; CSZ1 is first phase (reverse) of Coast shear zone deformation. (B) Final stage (D4 ) of transtensional denudation of orogen after emplacement of the younger phase of the Quottoon pluton; CSZ2 is second phase (normal) of Coast shear zone deformation, and SMZ is Shames mylonite zone. The reference surface is included to provide an estimate of the relative vertical displacement of the rocks between the D2 deformation and the end of the exhumation at about 50 Ma.

were emplaced within an increasingly transtensional environment. By 59–52 Ma, normal faulting leading to extension in an arc-parallel direction, documented in the region of the Quottoon plutonic complex and the Khyex sill complex, and in an arc-normal direction as recorded in the Coast shear zone completely dominated deformation within and at the boundaries of the batholith (Fig. 9B). Toward the end of this period between 55 and 52 Ma, large volumes of tonalite magma that comprise the youngest part of the Quottoon plutonic complex and the Kasiks pluton to the east (Hollister and Andronicos, 1997)

were emplaced. Also after 55 Ma but before 50 Ma, top-down-to-the-northeast and -east normal displacements within the Shames mylonite zone (Heah, 1991) and the Portland Canal shear zone (Crawford, unpublished data) took place along the eastern side of the batholith (Fig. 9B). The tectonic denudation of the batholith leading to its exhumation and cooling also happened at this time as documented by U–Pb sphene cooling ages from plutons at Sites 7, 8, 10 and Toon Lake (Table 1) and by 39 Ar=40 Ar ages on hornblende and biotite (Wood et al., 1991; Hollister, 1993). The presence of a magmatic fabric that shows no evidence of any solid-state deformation except possibly at pluton margins supports the interpretation that the tabular 59–55 Ma tonalite bodies of the Quottoon and Arden Lake plutonic complexes were emplaced into dilational zones associated with both ductile normal faults and sinistral transtensional shear zones active during magma emplacement. Despite their strongly deformed wall rocks there are no crosscutting relations between the igneous rocks and foliations in the country rock shear zones. In its eastern contact zone, the host rocks of the 55 Ma phase of the Quottoon plutonic complex, the thickest of all these bodies, were heated and partially melted. Sinistral ductile deformation within this contact zone in the presence of melt provides additional evidence for pluton emplacement and coeval shear zone deformation. Folded and flattened syntectonic felsic dikes in the country rocks at the eastern margin of the Quottoon plutonic complex suggest near field compression due to inflation of the pluton during emplacement. We have seen no evidence to support the statement by Hollister and Andronicos (1997) that the Quottoon pluton intruded an active dextral transpressive shear zone during the waning stages of deformation. Plutons within the Khyex sill complex and their enclosed synplutonic dikes show a remarkable parallelism with respect to orientation of a regional Nand NE-dipping S2 foliation. Top-down-to-the-north and -northwest kinematic indicators, including welldeveloped mineral and stretching lineations, lead us to suggest that D2 deformation resulted from normal and sinistral transtensional displacements parallel to (NW) and slightly oblique to (NNW and N) the northwesterly trend of the orogen (see also Klepeis

M.L. Crawford et al. / Tectonophysics 312 (1999) 57–78

and Crawford, in press, and extensional fabrics described by Andronicos et al., 1998). The tabular pluton shapes and regional concordance between the pluton margins and the N- to NE-dipping pervasive S2 foliation, both features among the criteria cited by Paterson et al. (1989) for syntectonic igneous bodies, suggest that pluton emplacement was coeval with the D2 phase of deformation. The resulting transfer of crustal rocks parallel to the arc accompanied by vertical displacements during D2 appears to have moved crustal material laterally to make room for the plutons of this 67–63 Ma group and thus facilitated their emplacement. Additional observations that support this conclusion include an abundance of top-down-to-the-north kinematic indicators within a pre-full-crystallization-type shear bands within the Khyex sills and the fact that this high-temperature fabric can be traced directly into solid-state fabrics at the sill margins and in the host rock gneiss. The well-developed N- and NE-dipping fabric inherited from D1 and accentuated during D2 provided a strong structural anisotropy that may have served to control the shape of sites for magma accumulation. Because this foliation presently shows a gentle dip to the north where it is unaffected by later deformation, we suggest that it probably formed at a low angle. Tilting following formation may have altered the angle and direction of the dip but it is unlikely that S2 was initially steep. As a consequence we suggest that the emplacement of the 67–63 Ma sills resulted in crustal thickening. This record of batholith emplacement and changing kinematic patterns provides solutions to the room problem involved in creating space or moving material out of the way to permit magma accumulation into plutons. Hutton (1982) suggested that bends and offsets along large transcurrent faults could serve as sites for passive emplacement of plutons. Based on a correlation of pluton emplacement with faulting in California during the Mesozoic, Glazner (1991) and Tikoff and Teyssier (1992) identified settings in which plutonism was facilitated by strike-slip faulting. This faulting resulted in tectonic voids formed at releasing bends along major transcurrent faults (Glazner, 1991) or in bridges between P shears (Tikoff and Teyssier, 1992). Crawford and Crawford (1991) and Davidson et al. (1992) report pluton emplacement along thrust-sense shear zones active

75

in convergent tectonic settings. Hutton et al. (1990) and Grocott et al. (1994) suggested that pluton formation reflects the creation of favorable sites by large-scale extensional deformation. These studies, as well as our own, suggest that plutons form during crustal deformation that creates dilational sites within the crust as a result of a regional-scale response to plate interactions. Our study contributes additional insights into this issue of pluton accumulation in the middle to deeper crust. In contrast to the studies cited above, in the central Coast Mountains batholith the main phase of pluton emplacement, between 67 and 63 Ma, was not restricted to sites along regional faults. Rather it appears that room was created for magma accumulation into plutonscale sills and sill complexes by regionally pervasive extensional deformation. As the orogen continued to develop extension partitioned into large-scale shear zones and numerous small but still discrete shears and normal faults. As strain became more localized into discrete structures after 60 Ma, so too did the sites of magma accumulation closely related to the areas of high strain that favored passive magma emplacement. How much of the shift from a more regionally distributed deformation and accumulation of igneous rocks to more localized deformation and pluton formation, leading to large volume individual plutons, was enhanced by changes in rheology of the crust associated with the increasing volume of melt in the orogen is unknown.

7. Conclusions The structure, timing, and kinematics of deformation that accompanied the emplacement of the central part of the Coast Mountains batholith of northern coastal British Columbia near 54.5ºN result from pluton emplacement in a tectonic setting characterized by convergence between the Farallon=Kula and North American plates. U–Pb isotopic, structural, and kinematic data indicate that deformation within the batholith involved normal faults with arc-parallel to oblique displacements as well as arc-parallel sinistral transtensional faults. These displacements created dilational sites into which plutons were emplaced beginning at ¾67 Ma. Following a period of crustal thickening that accompanied emplacement of

76

M.L. Crawford et al. / Tectonophysics 312 (1999) 57–78

67–63 Ma plutons, the deep roots of the batholith were unroofed by ductile normal faults prior to 50 Ma. Pluton emplacement, crustal thickening, and the accompanying orogen parallel tectonic unroofing lasted from approximately 70 to 50 Ma. Coeval shear zones along the western and eastern margins of the batholith may have formed in response to the emplacement of plutons of the batholith, and helped facilitate batholith exhumation. The central Coast Mountains batholith between the Skeena River and Portland Inlet (Fig. 5) consists of three successively younger groups of igneous rocks. Each group of rocks shows strong differences in fabric texture and orientation, shape and geometry, and relationships with respect to different styles of deformation. The sequence of events we have recognized in the batholith is summarized in Table 2. The oldest group of plutons, ranging in age from 88 Ma to 71 Ma, comprises orthogneisses with fabrics identical to the oldest fabric preserved in the country rocks. This fabric shows top-up-to-the-south and -southwest, thrust-type displacements. The ¾67–60 Ma main phase bodies were emplaced during pervasive regional ductile deformation that resulted in a penetrative gently to moderately N-dipping fabric with top down-to-the-north normal displacements. These crosscut the older fabric and the 87–70 Ma plutons. Undeformed 59–52 Ma tonalite bodies including the volumetrically large and youngest tonalite phase of the Quottoon plutonic complex and the young tabular bodies in the Arden Lake plutonic complex represent the next major batholith phase. These plutons were emplaced into severalkilometer wide arc-parallel sinistral transtensional shear zones. These shear zones, which are oriented transverse to the NNW strike of the orogen provided dilational sites for pluton emplacement and simultaneously unroofed the thickened crustal welt. The shapes of the plutons were constrained both by synchronous regional deformation in the orogen and by the pre-existing fabric that created a strong crustal anisotropy. Similar but smaller shear zones with N-down sinistral oblique displacement host late granite pegmatite and aplite dikes. The youngest ¾51–50 Ma aplite and pegmatite dikes post-date all phases of ductile deformation. They were emplaced during the final phase of uplift, cooling and exhumation of the batholith. Pluton emplacement, crustal

thickening, and accompanying orogen-parallel extension culminated in unroofing of the mid-crustal roots of the orogen between 55 Ma and 50 Ma. Coeval shear zones along the western and eastern margins of the batholith related to the uplift of the batholithic crustal welt truncated the plutons of the batholith at this time.

Acknowledgements NSF grants 93-04321 to M.L. Crawford and EAR 93-03824 to G.E. Gehrels made some of this work possible. Other results were obtained as part of the ACCRETE collaborative program funded by the Continental Dynamics Program of NSF (EAR 9527395 to M.L. Crawford and EAR 95-26263 to G.E. Gehrels). L.S. Hollister and B. Douglas had previously mapped in parts of the area covered in this report. In addition, K.A. Klepeis thanks C. Andronicos, C. Davidson and L.S. Hollister for helpful discussions and a visit to the part of the batholith nearest the Skeena River, south of our study area. We appreciate the careful reviews by K. McCaffery, M. Rusmore and W.A. Crawford that considerably improved this paper. Finally thanks to C. Simpson and K. Benn who organized the symposium that resulted in this publication.

References Andronicos, C.L., Chardon, D.H., Hollister, L.S., 1998. Synchronous sill emplacement, extensional deformation, and granulite facies metamorphism within the Coast plutonic complex, British Columbia. Geol. Soc. Am. Abstr. Programs 30, 351– 352. Berg, H.C., Elliot, R.L., Koch, R.D., 1988. Geologic map of the Ketchikan and Prince Rupert quadrangles, southeastern Alaska. U.S. Geological Survey Miscellaneous Investigations Map I-1807, scale 1 : 250,000. Chardon, D., Andronicos, C.L., 1997. The Central Gneiss Complex: An asymmetrical extensional gneiss dome bounded by the Coast shear zone. Geol. Soc. Am. Abstr. Programs 29, A-84. Cook, R.D., Crawford, M.L., 1994. Exhumation and tilting of the western metamorphic belt of the Coast orogen in southern southeastern Alaska. Tectonics 13, 528–531. Cook, R.D., Crawford, M.L., Omar, G.I., Crawford, W.A., 1991. Magmatism and deformation, southern Revillagigedo Island, southeastern Alaska. Geol. Soc. Am. Bull. 103, 829–841.

M.L. Crawford et al. / Tectonophysics 312 (1999) 57–78 Crawford, M.L., Crawford, W.A., 1991. Magma emplacement in a convergent tectonic orogen, southern Revillagigedo Island, southeastern Alaska. Can. J. Earth Sci. 28, 929–938. Crawford, M.L., Hollister, L.S., 1982. Contrast of metamorphic and structural histories across the Work Channel lineament, Coast Plutonic Complex, British Columbia. J. Geophys. Res. 87, 3849–3860. Crawford, M.L., Kraus, D.W., Hollister, L.S., 1979. Petrologic and fluid inclusion study of calc-silicate rocks, prince Rupert, British Columbia. Am. J. Sci. 279, 1135–1159. Crawford, M.L., Hollister, L.S., Woodsworth, G.J., 1987. Crustal deformation and regional metamorphism across a terrane boundary, Coast Plutonic Complex, British Columbia. Tectonics 6, 343–361. Davidson, C., Hollister, L.S., Schmid, S.M., 1992. Role of melt in the formation of a deep-crustal compressive shear zone: the MacLaren Glacier metamorphic belt, south central Alaska. Tectonics 11, 348–359. Douglas, B.J., 1986. Deformational history of an outlier of metasedimentary rocks, Coast Plutonic Complex, British Columbia, Canada. Can. J. Earth Sci. 23, 813–826. Evenchick, C.A., Crawford, M.L., McNicoll, V.J., Currie, L.D., O’Sullivan, P.B., 1999. Early Miocene or younger normal faults and other Tertiary structures in west Nass River map area, northwest British Columbia, and adjacent parts of Alaska. Current Research 1999-A, Geological Survey of Canada, pp. 1–11. Gehrels, G.E., Saleeby, J.B., 1987. Geologic framework, tectonic evolution, and displacement history of the Alexander terrane. Tectonics 6, 151–173. Gehrels, G.E., McClelland, W.C., Samson, S.D., Patchett, P.J., 1991a. U–Pb geochronology of the detrital zircons from a continental margin assemblage in the northern Coast Mountains, southeastern Alaska. Can. J. Earth Sci. 28, 1285–1300. Gehrels, G.E., McClelland, W.C., Samson, S.D., Patchett, P.J., 1991b. U–Pb geochronology of late Cretaceous and early Tertiary plutons in the northern Coast Mountains batholith. Can. J. Earth Sci. 28, 899–911. Glazner, A.F., 1991. Plutonism, oblique subduction, and continental growth: an example from the Mesozoic of California. Geology 19, 784–786. Grocott, J., Brown, M., Dallmeyer, R.D., Taylor, G.K., Treloar, P.J., 1994. Mechanism of continental growth in extensional arcs: an example from the Andean plate boundary zone. Geology 22, 391–394. Heah, T.S.T., 1991. Mesozoic Ductile Shear and Paleogene Extension along the Eastern Margin of the Central Gneiss Complex, Coast Belt, Shames River Area, near Terrace, British Columbia. MS thesis, University of British Columbia, Vancouver, 155 pp. Hollister, L.S., 1982. Metamorphic evidence for rapid (2 mm=yr.) uplift of a portion of the Central Gneiss Complex, Coast Mountains, British Columbia. Can. Mineral. 20, 319–332. Hollister, L.S., 1993. The role of melt in the uplift and exhumation of orogenic belts. Chem. Geol. 108, 31–48. Hollister, L.S., Andronicos, C.L., 1997. A candidate for the Baja

77

British Columbia fault system in the Coast plutonic complex. GSA Today 7, 1–7. Hollister, L.S., Crawford, M.L., 1990. Crustal formation at depth during continental collision. In: Salisbury, M.H., Fountain, D.M. (Eds.), Exposed Cross-Sections of the Continental Crust. Kluwer, Norwell, MA, pp. 215–226. Hollister, L.S., Diebold, J., Lowe, C., Rohr, K., Humphreys, C., Smithson, S.B., Crawford, M.L., Crawford, W.A., Clowes, R.M., Ellis, R.M., Trehu, A., Gehrels, G.E., 1994. Seismic image of the deep crustal structure of the Coast orogen of southeast Alaska and British Columbia. Geol. Soc. Am. Abstr. Prog. 26, 146. Hutchison, W.W., 1982. Geology of the Prince Rupert–Skeena map area, British Columbia. Geol. Surv. Can. Mem. 394, 116 pp. Hutton, D.H.W., 1982. A tectonic model for the emplacement of the Main Donegal Granite, NW Ireland. J. Geol. Soc. London 139, 615–631. Hutton, D.H.W., 1988. Igneous emplacement in a shear-zone termination: The biotite granite at Strontian, Scotland. Geol. Soc. Am. Bull. 100, 1392–1399. Hutton, D.H.W., Dempster, T.J., Brown, P.E., Becker, S.D., 1990. A new mechanism of granite emplacement; intrusion in active extensional shear zones. Nature 343, 452–455. Ingram, G.M., Hutton, D.H.W., 1994. The Great Tonalite Sill; emplacement into a contractional shear zone and implications for Late Cretaceous to early Eocene tectonics in Southeastern Alaska and British Columbia. Geol. Soc. Am. Bull. 106, 715– 728. Klepeis, K.A., Crawford, M.L., in press. High-temperature arcparallel normal faulting and transtension at the roots of an obliquely convergent orogen. Geology, 27. Klepeis, K.A., Crawford, M.L., Gehrels, G.E., 1998. Structural history of the crustal-scale Coast shear zone near Portland Inlet, SE Alaska and British Columbia. J. Struct. Geol. 20, 883–904. McClelland, W.C., Gehrels, G.E., 1990. Geology of the Duncan Canal shear zone: evidence for Early to Middle Jurassic deformation of the Alexander terrane, southeastern Alaska. Geol. Soc. Am. Bull. 102, 1378–1392. McClelland, W.C., Gehrels, G.E., Samson, S.D., Patchett, P.J., 1992. Structural and geochronologic relations along the western flank of the Coast Mountains batholith: Stikine River to Cape Fanshaw, central SE Alaska. J. Struct. Geol. 14, 475– 489. Monger, J.W.H., Price, R.A., Templeman-Kluit, D.J., 1982. Tectonic accretion and the origin of the two major metamorphic and plutonic welts in the Canadian Cordillera. Geology 10, 70–75. Paterson, S.R., Fowler, T.K., 1993. Extensional pluton-emplacement models: do they work for large plutonic complexes? Geology 21, 781–784. Paterson, S.R., Vernon, R.H., Tobisch, O.T., 1989. A review of criteria for the identification of magmatic and tectonic foliations in granitoids. J. Struct. Geol. 11, 349–363. Petford, N., Atherton, M.P., 1992. The role of extension in the generation, ascent, and emplacement of peraluminous melts,

78

M.L. Crawford et al. / Tectonophysics 312 (1999) 57–78

in thickened continental crust; the Cordillera Blanca Batholith, Peru. 29th Int. Geol. Congr. Abstr. 29, 519. Rohr, K.M.M., Dietrich, J.R., 1992. Strike-slip tectonics and development of the Tertiary Queen Charlotte Basin, offshore Western Canada: evidence from seismic reflection data. Basin Res. 4, 1–19. Rohr, K.M.M., Furlong, K.P., 1995. Ephemeral plate tectonics at the Queen Charlotte triple junction. Geology 23, 1035–1038. Rubin, C.M., Saleeby, J.B., 1992. Tectonic history of the eastern edge of the Alexander terrane, southeast Alaska. Tectonics 11, 586–602. Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial lead isotope evolution by a two-stage model. Earth Planet. Sci. Lett. 26, 207–221. Thomas, J.A. 1998. Field, Petrographic, Geochemical, and Isotopic Evidence for Magma Mixing and AFC Processes in the Quottoon Igneous Complex, Northwestern British Columbia and Southeastern Alaska. MS Thesis, Virginia Polytechnic

Institute and State University. Tikoff, B., Saint Blanquat, M., 1997. Transpressional shearing and strike-slip partitioning in the Late Cretaceous Sierra Nevada magmatic arc, California. Tectonics 16, 442–459. Tikoff, B., Teyssier, C., 1992. Crustal-scale, en echelon ‘P-shear’ tensional bridges: a possible solution to the batholithic room problem. Geology 20, 927–931. Tobisch, O.T., Saleeby, J.B., Renne, P.R., McNulty, B., Weixing, T., 1995. Variations in deformation fields during development of a large-volume magmatic arc, central Sierra Nevada, California. Geol. Soc. Am. Bull. 107, 148–166. Tucker, R.D., Ashwal, L.D., Zinner, E.K., 1998. Slow cooling of deep crustal granulites and Pb-diffusion in zircon. Geol. Soc. Am. Abstr. Prog. 30, A-213. Wood, D.J., Stowell, H.H., Onstott, T.C., Hollister, L.S., 1991. 40 Ar=39 Ar constraints on the emplacement, uplift, and cooling of the Coast Plutonic Complex sill, southeastern Alaska. Geol. Soc. Am. Bull. 103, 849–860.