Strain partitioning in an obliquely convergent orogen, plutonism, and synorogenic collapse: Coast Mountains Batholith, British Columbia, Canada

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1 TECTONICS, VOL. 22, NO. 2, 1012, doi: /2001tc001312, 2003 Strain partitioning in an obliquely convergent orogen, plutonism, and synorogenic collapse: Coast Mountains Batholith, British Columbia, Canada Christopher L. Andronicos, 1 Dominique H. Chardon, 2 and Lincoln S. Hollister Department of Geosciences, Princeton University, Princeton, New Jersey, USA George E. Gehrels Department of Geosciences, University of Arizona, Tucson, Arizona, USA Glenn J. Woodsworth Geological Survey of Canada, Vancouver, British Columbia, Canada Received 28 June 2001; revised 2 August 2002; accepted 16 September 2002; published 15 April [1] We describe the crustal structure of the Coast Mountains batholith between 54 and 55 N, within the Canadian Cordillera, with emphasis on emplacement of the 7 km thick Kasiks sill complex (KSC). Kinematic patterns that developed during emplacement of the KSC are the result of interactions between magma transport, magma accumulation and regional deformation. The sills were emplaced during NW directed normal shearing and flattening of country rocks that host the KSC. A 2 km thick shallowly NE dipping mylonite zone cuts the eastern side of the KSC. Kinematic indicators within the mylonite zone record top to the east normal displacements. Structural analysis shows that mylonite formation occurred during subvertical shortening and east-northeast, subhorizontal extension. U/Pb zircon age dates show that ENE directed normal shearing along the eastern side of the KSC and WNW directed normal shearing within the KSC occurred contemporaneously between 54 and 51 Ma, indicating strong strain partitioning between the mylonite and the KSC. This pattern of strain partitioning is interpreted to have been driven by return flow of melt-laden crust in response to tectonic denudation of the upper crust. Seismic profiling shows that many of these structures extend to mid and lower crustal depths. Comparison of our results with other regions within the Canadian Cordillera indicates that orogen-scale right-lateral strike-slip faults deformed synchronously with wide spread magmatism and formation of extensional gneiss domes. Thus the 1 Department of Geological Sciences, University of Texas at El Paso, El Paso, Texas, USA. 2 European Centre for Research in Environmental Geosciences, CNRS, Université Aix-Marseille III, Aix-en-Provence, France. Copyright 2003 by the American Geophysical Union /03/2001TC crustal structure of the Coast Mountains batholith was the result of early Tertiary batholith construction during dextral oblique convergence and synorogenic collapse. INDEX TERMS: 8102 Tectonophysics: Continental contractional orogenic belts; 9604 Information Related to Geologic Time: Cenozoic; 9350 Information Related to Geographic Region: North America; KEYWORDS: extension, plutonism, Canada, exhumation, tectonics, transtension. Citation: Andronicos, C. L., D. H. Chardon, L. S. Hollister, G. E. Gehrels, and G. J. Woodsworth, Strain partitioning in an obliquely convergent orogen, plutonism, and synorogenic collapse: Coast Mountains Batholith, British Columbia, Canada, Tectonics, 22(2), 1012, doi: / 2001TC001312, Introduction [2] Cordilleran batholiths are extensive belts of intermediate calc-alkalic plutons that typically formed on continental crust above subduction zones. The mechanisms by which these granitic batholiths form continue to pose several major problems in modern geology. Foremost is the problem of quantifying the rates and processes of crustal growth vs. those of recycling in arc environments [e.g., Hamilton, 1988]. [3] The study area (Figure 1) is within and just east of the largest batholithic complex of the world, the late Cretaceous-early Tertiary portion of the Coast Plutonic Complex (CPC) of southeast Alaska, British Columbia, and Washington State; this portion of the CPC is called the Coast Mountains batholith (CMB) [Hollister and Andronicos, 2000] (Figure 1). The CMB is over 2000 km long and forms the largest plutonic belt in the Canadian Cordillera. Eocene and Paleocene age plutons account for most of the volume of plutonic rock within the CMB (Figure 1) and cover a surface area of more than 75,000 km 2. The early Tertiary portion of this magmatic pulse is arguably one of the largest magmatic events in Earth s history. For comparison, the total area covered by plutons of all ages in the Sierra Nevada batholith is 40,000 km 2. The present study documents the evolution of one complex of early Tertiary sills within the CMB, called the Kasiks sill complex. Its

2 7-2 ANDRONICOS ET AL.: EXTENSION IN THE COAST PLUTONIC COMPLEX Figure 1. Geologic map of region between N and 55 N in the Coast Plutonic Complex. Line marked DA is axis of gneiss dome. Lines of sections in Figure 3 are also shown. Inset map shows the Coast Mountains batholith between 49 and 60 N, with study area indicated by box. exposed surface area is about 1000 km 2, but on the basis of the ACCRETE seismic results [Morozov et al., 1998, 2001], the shallowly dipping sill complex may have a surface area of 3000 km 2. Its thickness of 7 km within a crust 30 km thick suggests that it occupies about one quarter of the present continental crust. [4] This paper addresses the questions of the tectonic framework prior to, during and following intrusion of the Kasiks sill. Our results provide tight constraints on the processes active during the construction and generation of this large igneous complex. These constraints are essential to know in order to answer the fundamental questions regarding the origin of batholiths. Our data point to the conclusion that the Kasiks sill intruded during early Eocene extension of a melt-weakened crust. This extension is correlated with a reorganization of plate kinematics during the early Tertiary [Lonsdale, 1988; Engebretson et al., 1985] and with concurrent extension previously documented [Struik, 1992; Ewing, 1980] across the Canadian cordillera. [5] The early Tertiary extension resulted in (1) the juxtaposition of the CMB with low-grade supracrustal rocks across a 2 km thick east to northeast dipping normal shear zone, (2) intrusion of the Kasiks sill due to return flow driven by extensional denudation of the upper crust, and (3) subvertical flattening during exhumation of midcrustal rocks to shallow crustal levels between 55 and 48 Ma. The crustal structure determined from the ACCRETE seismic experiment [Morozov et al., 1998, 2001] is mainly the result of this terminal tectonic episode. These results highlight the importance of crustal extension in the construction of the Coast Mountains batholith. 2. Geologic Setting [6] The Coast Plutonic Complex (CPC) of the western Canadian Cordillera stretches from southeastern Alaska to northwestern Washington State striking parallel to the north-

3 ANDRONICOS ET AL.: EXTENSION IN THE COAST PLUTONIC COMPLEX 7-3 west trending structural grain of the orogenic belt (Figure 1). There is a west to east age progression of the plutons across strike of the CPC. Jurassic and Early Cretaceous plutons are found along the western side of the CPC, middle Cretaceous (100 to 86 Ma) plutons are found in the central part of the CPC west of the Coast shear zone, and late Cretaceous to Eocene (86 to 50 Ma) plutons are found to the east of the Coast shear zone [van der Heyden, 1992]. The 86 to 50 Ma plutons and their country rocks are referred to here as the Coast Mountains batholith, following Hollister and Andronicos [2000] (Figure 1). Most of the plutons that make up the Coast Plutonic complex are calc-alkaline, ranging from tonalite to granodiorite, although diorites, gabbros, and true granites occur locally [Hutchison, 1982a]. [7] The study area, located between 54 and 55 N, within the Coast Plutonic Complex, is dominated by a belt of amphibolite to granulite facies migmatitic gneiss that forms the country rock to large tonalite-granodiorite batholiths of the Coast Plutonic Complex [Hutchison, 1982a]. The lithologies are described in detail by Hollister and Andronicos [2000]. This gneiss belt is called the Central Gneiss Complex by Hutchison [1970, 1982a]. The predominant supracrustal rock types are rusty weathering migmatite, which is locally rich in sillimanite and garnet; gray weathering migmatite, which contains variable amounts of hornblende and biotite; and amphibolites that locally contain orthopyroxene and clinopyroxene, particularly near the margins of the voluminous late Cretaceous and early Tertiary batholiths [Hollister, 1975, 1982]. The rusty weathering units are metaclastic rocks whereas the gray weathering units are metavolcanic rocks and orthogneiss bodies [Douglas, 1986; Hill, 1984; Hollister and Andronicos, 2000]. The amphibolites are metamorphosed basalt flows, dikes, and sills. [8] Within the study area, batholith sized plutons, including and east of the Quottoon pluton, range in age from 86 to 50 Ma. The intrusion of the largest volume of these batholiths spans the relatively narrow time interval 60 to 50 Ma and includes the Quottoon pluton, the Kasiks sill and parts of the Ponder pluton (Figure 1). The Quottoon pluton is a medium grained hornblende biotite tonalite that most authors concluded intruded during contractional deformation across the Coast shear zone [Andronicos et al., 1999; Hollister and Andronicos, 1997; Ingram and Hutton, 1994; Klepeis et al., 1998]. However, in a contrasting interpretation, Klepeis and Crawford [1999] concluded that the Quottoon pluton was emplaced during regional sinistral transtension. Al in hornblende geobarometry and study of metamorphic country rocks adjacent to the Quottoon pluton suggest that it was emplaced at pressures between 4 and 6 kbar, where studied near the Skeena River [Kenah and Hollister, 1983; Lappin and Hollister, 1980; Hollister et al., 1987]. Thomas and Sinha [1999] describe the petrology and geochemistry of the Quottoon pluton. [9] The Ponder pluton is a complex of late Cretaceous and early Tertiary plutons that range in composition from tonalite to granite [Gareau et al., 1997; Hutchison, 1982b; Sisson, 1985]. Pressure estimates from study of the metamorphic country rocks around the Ponder pluton suggest emplacement pressures of 5 kbar along its western side [Hill, 1984; Sisson, 1985], whereas the eastern side of the pluton was emplaced at pressures between 2 and 3 kbar [Sisson, 1985; Hollister et al., 1987]. These pressure estimates and the silllike geometry imply the Ponder pluton is a shallowly northeast dipping sheet with its base exposed on its western side and roof exposed on its eastern side (Figure 1). [10] The Kasiks sill complex (Figure 1) is composed of meter- to kilometer-scale sills that range in composition from diorite to tonalite [Hutchison, 1982a, 1982b; Crawford et al., 1987; Hollister and Andronicos, 2000]. Al in hornblende barometry and study of the metamorphic country rocks intruded by the Kasiks sill complex suggest it was emplaced at pressures between 4 and 5 kbar [Hollister, 1982; Hollister et al., 1987]. Most of the plutons within the study area contain magmatic foliations; locally developed solid state foliations are associated with high strain zones. Foliations within the plutons are generally concordant with those in the metamorphic country rocks. [11] Rocks to the east of the Ponder pluton include Jurassic and Cretaceous sedimentary rocks of the Bowser Basin, Jurassic volcanic rocks of the Hazelton group, and metamorphic rocks of the Kitsumkalum block [Gareau et al., 1997] (Figure 1). These rocks are generally metamorphosed to the greenschist facies, except near plutons where contact metamorphism reaches the amphibolite facies [Sisson, 1985]. Plutonic rocks east of the Ponder pluton include Paleozoic plutons, Jurassic plutons and shallow level late Cretaceous and early Tertiary Plutons. [12] Rocks to the west of the Quottoon pluton include schist, gneiss and amphibolite intruded by mid-cretaceous batholiths (Figure 1). Many of these batholiths contain igneous epidote and garnet and were intruded at 7 9 kbar [Crawford and Hollister, 1982; Hammarstrom and Zen, 1986; Hollister et al., 1987]. The area around Prince Rupert contains an inverted metamorphic sequence that has been interpreted as a west verging thrust belt rooted in the lower crust [Crawford et al., 1987]. These rocks are separated from the Early Tertiary core of the Coast Plutonic Complex by the Coast shear zone, a crustal-scale shear zone that has had a complex and protracted deformation history [Andronicos et al., 1999; Chardon et al., 1999; Ingram and Hutton, 1994; Klepeis et al., 1998; Stowell and Hooper, 1990]. The Coast shear zone is associated with a major topographic lineament called the Coast Ranges Megalineament in Alaska [Brew and Ford, 1978] and the Work Channel Lineament in Canada [Hutchison, 1982b]. The uplift history of the Coast Plutonic Complex between Terrace and Prince Rupert, British Columbia (Figure 1) has been studied by Douglas [1986], Harrison and Clark [1979], Heah [1991b], Hill [1984], Hollister [1979, 1982], Hollister and Andronicos [1997], Crawford et al. [1987], Hollister et al. [1987], Klepeis and Crawford [1999], and Sisson [1985]. Hollister [1979, 1982] suggested that exhumation of the Coast Mountains batholith was driven by erosional denudation during contraction. Crawford et al. [1987] presented a similar model in which exhumation was driven by uplift on subvertical shear zones on the margins of the batholith complex. Heah [1991b] concluded that uplift was in part caused by crustal extension. Hollister and Andronicos [1997] pointed

4 7-4 ANDRONICOS ET AL.: EXTENSION IN THE COAST PLUTONIC COMPLEX Figure 2. Geologic map and cross section of Toon Lake pluton area. (a) Geologic map showing domains described in text. (b) Cross section. (c, e, f, and g) Lower hemisphere equal-area projections of poles to foliation (black dots) and mineral lineations (white circles). (d) Lower hemisphere equal-area projection of poles to shear bands. See text for discussion. out the role of extension in the emplacement of voluminous Eocene batholiths. Klepeis and Crawford [1999] supported these earlier models and suggested that emplacement of large sills in the core of the batholith was synchronous with extensional denudation between 67 and 50 Ma. In the present paper, our data show that the uplift, exhumation, and plutonism occurred between 54 and 48 Ma. 3. Structure [13] The structural geology of the region between 54 and 55 N within Figure 1 is described in reference to five study areas. Each of these areas records a distinct part of the overall deformation history. Five separate episodes of ductile deformation have affected the high-grade core of the batholith. These deformation events are described by first discussing an area away from the main body of the Kasiks sill complex. The overprinting relationships in this area give constraints on the deformation history of regions removed from large Eocene batholiths. This is followed by descriptions of map areas on the western and eastern sides of the Kasiks sill complex that are dominated by structures produced in the early Eocene Toon Lake Area [14] The area from the 72 Ma Toon Lake pluton [Crawford et al., 1999] toward the east shows striking examples of map-scale overprinting relationships (Figures 2a and 2b). S 1 in the Toon Lake area is a west-northwest striking, moderately northeast dipping foliation (Figures 2a and 2f) defined by a gneissic banding. L 1 mineral lineations plunge shallowly north-northwest. Going west toward the Toon Lake pluton, S 1 is cut and transposed into northwest striking, steeply dipping foliation planes (S 2 ) (Figures 2a, 2f, and 2e). The eastern portion of the Toon Lake pluton has a solidstate foliation that is concordant with the gneissic foliation in the adjacent country rocks. Going west and up in elevation, within the Toon Lake pluton, the S 2 foliation is refolded and transposed into a moderate to shallowly north

5 ANDRONICOS ET AL.: EXTENSION IN THE COAST PLUTONIC COMPLEX 7-5 Figure 3. Photographs of outcrop-scale structures. (a) Asymmetric clast indicating north directed shearing from east side of Figure 2. (b) Symmetrical boudins in roof region of Kasiks sill with conjugate shear bands above Skeena River on west side of pluton. (c) Asymmetric boudins developed in synplutonic dike, shear sense is top to the south, location same as in Figure 3b. (d) Fold hinges parallel to stretching lineation in constrictive shear zone adjacent to Kasiks sill complex in central part of Figure 5. dipping foliation (S 3 ) (Figures 2a and 2b) that is pervasively intruded by decimeter- to meter-scale tonalite sills. The same type of overprinting relationship is seen to the east of the Toon Lake pluton, where S 1 is overprinted by an eastwest striking foliation with moderate to shallow northern dips (Figure 2g), identical in orientation to S 3 within the Toon Lake pluton (Figures 2a and 2b). It is interesting to note that another 50 m of erosion off of the Toon Lake pluton would have erased any evidence for S 3 overprinting S 2 within the Toon Lake pluton (Figures 2a and 2b). [15] Each of these geometric domains is associated with distinct changes in kinematics. S 1 is associated with penetrative ductile top to the south shearing (Figures 2a and 2b), indicated by sigma porphyroclasts and shear bands. Kinematics within the steeply dipping gneisses (S 2 ) on the eastern side of the Toon Lake pluton (Figure 2d) are complex with both dextral and sinistral shear bands occurring. Geometrically, the shears appear to form conjugate sets that accommodate southwest-northeast shortening (Figure 2d). The geometry of the shears combined with the great circle distribution of lineations suggests predominately flattening strain along the margin of the pluton. Shear bands within the Toon Lake pluton consistently indicate dextral shearing parallel to the margin of the pluton, also consistent with northeast-southwest subhorizontal shortening. [16] Foliations within the steeply dipping gneiss and amphibolite along the eastern side of the Toon Lake pluton define a flower geometry with foliations dipping inward (Figure 2b). Foliations within the Toon Lake pluton are generally concordant with those in the country rocks, and solid state fabrics are well developed within the pluton. The concordance of the fabrics between the pluton and the adjacent gneisses and lack of overprinting relationships suggest that fabrics within the pluton developed contemporaneously with those in the country rocks. [17] These observations suggest the following deformation history during the development of S 1 and S 2 near the Toon Lake pluton. The first stage of deformation consisted of top to the south reverse shear across shallow fabrics (S 1 ). Then S 2 developed during southwest-northeast subhorizontal shortening, with a dextral component. Top to the south reverse deformation on S 1 may have continued as S 2 developed. The kinematic compatibility between top to the south shearing and dextral shearing within the pluton, similarity of lineation orientation, and similar shortening directions suggest that S1 and S2 developed as the result of progressive deformation, although they could record separate deformation events. Similar structures are described in regions farther to the south, near the Skeena River by Andronicos et al. [1999] and Hollister and Andronicos [2000], who concluded they formed during partitioned dextral transpression. [18] Deformation associated with S 3 is distinctly different in its kinematics. Sigma-type porphyroclasts, shear bands and asymmetric boudins all indicate penetrative top to the north shearing parallel to the shallowly north plunging stretching lineation (Figure 3a). The sills associated with

6 7-6 ANDRONICOS ET AL.: EXTENSION IN THE COAST PLUTONIC COMPLEX the S 3 foliation lack a solid-state foliation, and sill geometry is closely associated with ductile deformation features in the country rocks. For instance, the shear bands that cut through the country rocks to the margins of the sills offset the sill walls; but there is no evidence for recrystallization of minerals within the sills. Furthermore, feldspar and hornblende porphyroclasts are tabular suggesting they have not been recrystallized following magmatic flow [Patterson et al., 1989]. In addition, magmatic alignment of tabular feldspar and hornblende define a magmatic mineral lineation within the sills that parallels the lineation in the country rocks. Finally, sills are only intruded into rocks that contain the S 3 fabric. We conclude, on the basis of these features, that injection of the decimeter- and meter-scale sills was synkinematic with the north directed shearing. [19] To summarize the deformation history of the Toon Lake area, early top to the south displacement on S 1 predated but probably overlapped dextral displacements within the 72 Ma Toon Lake pluton and shortening at its margin. This early phase of deformation was overprinted by top to the north displacement and injection of meter-scale tonalite sills associated with S Kasiks Sill Complex Large-Scale Deformation Pattern [20] The Kasiks sill complex is located in the center of the Coast Mountains batholith and is associated with granulite facies metamorphism [Hollister, 1982]. In this section we describe the structures within and surrounding the sill complex and how they relate to the overall deformation history of the region. [21] On the western side of the complex, the contacts of the northern part of the pluton dip toward the west (Figures 1, 4, and 5). Toward the south, the contacts bend around and have approximately east-west strikes (Figure 1). This geometry defines a synform-like depression within the country rock gneiss along the western side of the sill complex (Figure 1). The eastern side of the sill complex has more consistently oriented contacts with west-northwest strikes and moderate to steep northerly dips (Figures 1 and 6). Mineral lineations throughout the sill complex and adjacent country rocks are shallowly to moderately north-northwest plunging except along the eastern side of the sill complex where east-west trending lineations occur (Figure 6). [22] The base of the sill complex is moderately north dipping at its most southwestern side (Figure 1). Going to the east, the contact bends around to north-northeast striking with steep to vertical dips. Farther to the east, the base of the pluton bends back around to northwest striking with moderate northern dips (Figure 1). This geometry defines an open antiform (Figure 7c). The eastern part of the sill complex is more tabular and is essentially a north dipping layer cake of interlayered sills and gneissic country rocks (Figures 6 and 7c). [23] The internal structure of the sill complex is geometrically less complicated. Magmatic and solid state foliations are subparallel and dip toward the north throughout most of the complex. The exception to this is along the western margin of the sill complex where magmatic foliations are subparallel to its north striking, west dipping margin (Figures 4 and 5). Magmatic lineations defined by elongate mafic schlieren, enclaves and tabular feldspar and hornblende crystals are subparallel to solid-state lineations. [24] Study of the geometry of minor shear zones, boudins, sill, and dike networks; orientation of folds; and orientations of foliations and lineations were used to estimate the finite strain state around the sill complex. The boudins in country rock gneiss are best developed where orthogneiss and paragneiss are interlayered with calc-silicates and amphibolites, providing layering with distinct competence contrasts. Strain ellipsoid shapes estimated from the amphibolite and calcsilicate boudins around the sill complex are generally of the flattening type (Figure 8), even where the boudins are strongly asymmetrical. Individual boudins are slightly elongate parallel to mineral lineations in the country rocks. K values and shape of the ellipses (in map view) are plotted on Figure 8 so that variations in strain around the sill complex can be easily visualized. A K value of 1 indicates an ellipsoid of the plane strain type, values <1 are pancake shaped, and values >1 indicate cigar shaped ellipses [Flinn, 1979]. It should be noted that the K value does not take into account volume changes, so it best describes ellipsoid shapes independent of dilatation, not necessarily strain path Western Kasiks Sill Complex [25] At the roof of the sill complex, on its western side where the contacts strike east-west, shear bands indicate both top to the north and top to the south senses of displacement (Figure 3). The shears have a conjugate geometry and occur with nearly symmetrical pancake shaped boudins (Figure 3b). However, shears with an asymmetry indicative of top to the south sense of displacement predominate and occur within the metamorphic country rocks and within the sills where top to the north shears are absent (Figure 3c). In addition, overprinting relationships are mutually crosscutting suggesting that both top to the north and top to the south shearing were occurring synchronously (Figure 3b). These relationships suggest that the north directed shears are antithetic to south directed sense of displacement in an area dominated by flattening stains. [26] In the most northern area studied in the Kasiks sill complex, foliations are variably oriented (Figures 5a and 5b). Away from the pluton, shallowly north dipping foliations with north plunging lineations show a superposition of early top to the south sense of shear and later top to the north sense of shear (Figures 5a and 5e); this is similar to that seen adjacent to the eastern contact of Toon Lake pluton. In addition, wherever the foliation is steeply dipping, boudins have a distinctly triclinic symmetry. In these areas, fold axes are aligned subparallel to a prominent stretching lineation and have sheath-like geometry (Figure 3d). Many of these zones have a synform-like geometry in cross section (Figure 5e). The asymmetry of the boudins and high angle shear bands suggests a north side down sense of displacement (Figure 5c) with a sinistral strike-slip component. The geometry of the folds and boudins suggests that these zones formed during horizontal constriction.

7 ANDRONICOS ET AL.: EXTENSION IN THE COAST PLUTONIC COMPLEX 7-7 Figure 4. Geologic map and cross section of area of interlayered country rock screens and sills on western margin of Kasiks sill. See text for discussion. [27] The western margin of the sill complex is subparallel to these narrow zones (Figure 5a). At the map scale, the contact is concordant with the country rock foliation. In contrast, at the outcrop scale, there is commonly 10 to 20 of discordance between the foliation in the country rocks and the contact. There is also a 2 3 m wide zone of grainsize reduction in the country rock gneisses adjacent to the contact. Fabrics in the pluton consist of alignment of tabular feldspar and hornblende, and they parallel the west dipping contact. Quartz grains are equant, filling interstitial voids between the large phenocrysts of plagioclase and hornblende, suggesting that these fabrics are magmatic in origin. There is a consistent deflection of the country rock foliation toward the pluton s contact. When viewed in a plane perpendicular to the contact and parallel to the lineation, the asymmetry of the deflection suggests a sinistral, south directed sense of displacement. Tabular dikes crosscut all of the deformation fabrics found in this area. Assuming the dikes represent tensile fractures, they define a shallowly northwest plunging stretching direction [Anderson, 1959] (Figures 5d and 8). [28] From the western contact of the sill complex toward the east (Figures 4a and 4f), several large screens of country rock strike north and are parallel to the contact. In the most western of these screens, asymmetric boudins and shear bands indicate sinistral, southwest side down sense of

8 7-8 ANDRONICOS ET AL.: EXTENSION IN THE COAST PLUTONIC COMPLEX Figure 5. complex. Geologic map and cross section of country rocks adjacent to northwest margin of Kasiks sill displacement subparallel to that seen at the western contact farther to the north (Figure 4c). In the central part of Figure 4a, foliations are east-west striking and have mostly down dip mineral lineations. Kinematic indicators consistently record north side up sense of displacement (Figure 4d). Farther east, mineral lineations are west plunging and down dip. Asymmetric folds, shear bands, and sigma-type porphyroclasts indicate consistent west side down normal displacement (Figure 4e). [29] In summary, the country rocks adjacent to the western side of the sill complex record early top to the south shearing overprinted by younger top to the north shearing. Narrow, north striking subvertical shear zones, with sinistral strike slip and north side down kinematics, parallel the contact of the sill complex. Sinistral shearing also affected the margin of the pluton and the country rock screens within the pluton. Intense flattening affects parts of the western margin of the sill complex.

9 ANDRONICOS ET AL.: EXTENSION IN THE COAST PLUTONIC COMPLEX 7-9 Figure 6. Geologic map and cross section of eastern portion of the Kasiks sill complex and Shames River mylonite zone. (a) Geologic map showing locations of domains discussed in text. Heavy dashed line marked am-gr is approximate location of amphibolite to granulite facies transition within the study area. (b) Lower hemisphere equal-area projection of poles to foliation (solid squares) and mineral lineations (open circles) in domain I. (c) Lower hemisphere equal-area projection of poles to dikes. Open circles are dikes with SW side down across their margins, and solid dots are dikes with northeast side down sense of shear across their margins. Xi, Yi, and Zi are calculated incremental strain directions. (d) Lower hemisphere equal-area projection of poles to foliation (solid squares) and mineral lineations (open circles) in domain II. (e) Lower hemisphere equal-area projection of poles to conjugate shear bands in domain II. Solid stars are sinistral, and open stars are dextral. Xi, Yi, and Zi are calculated incremental strain directions. (f) Lower hemisphere equal-area projection of poles to foliation (solid squares) and mineral lineations (open circles) in domain III. (g) Lower hemisphere equal-area projection of kinematic compression (open diamonds) and tension axis (solid dots) calculated from fault slip data. (h) Cross section across map area showing structural relationships Eastern Kasiks Sill Complex [30] The eastern side of the Kasiks sill complex (Figure 6) displays kinematic patterns that are distinctly different from those seen along its western side. Lineations plunge to the west-northwest along the western side (Figures 6a and 6b), those in the central part trend east-west and are subhorizontal (Figures 6a and 6d), and those on the eastern side plunge to the northeast (Figures 6a and 6f). Foliations, in contrast, are consistently west-northwest striking with north to northeastern dips (Figures 6a and 6h). As the trends of the mineral lineations change, fabrics display distinct structural styles and kinematics. We have therefore divided this area into three distinct geometric domains (Figure 6a). [31] A layer cake of north dipping sills, orthogneiss, and amphibolite dominates domains I and II (Figure 6h). The dominant fabric is a second-generation fabric S 2 as attested by rarely preserved outcrop scale overprinting relationships of S 2 on S 1 (Figures 9a and 9b). These

10 7-10 ANDRONICOS ET AL.: EXTENSION IN THE COAST PLUTONIC COMPLEX Figure 7. (a) ACCRETE seismic profile modified from Morozov et al. [1998]. Heavy lines are picked from refraction profile, and thin lines are reflections from vertical incidence data set. (b) Geologic cross section across gneiss dome in northern part of Figure 1. (c) Geologic cross section across the Kasiks sill complex. Inset shows location of sections. relationships show that a steeply east dipping north-northwest striking fabric predated the development of S 2. This orientation of fabrics is common south of the Skeena River (Figure 1) and is associated with dextral east side up transpressive shearing (Figure 8) [Andronicos et al., 1999; Hollister and Andronicos, 2000]. This overprinting relationship is generally consistent with that seen at Toon Lake, if S 2 at Toon Lake is equivalent to S 1 in this area. [32] Shear bands, asymmetric boudins, and sigma- and delta-type porphyroclasts in domain I indicate west to northwest side down normal sense displacement (Figures 6a and 6b). In addition to these highly asymmetric fabrics, features such as chocolate tabulate boudinage are common (Figure 9c). The development of these structures is clearly associated with the emplacement of the sills as indicated by the fact that the boudin necks are invaded by tonalite suggesting the sills were partly molten during the deformation (Figure 9c). [33] Domain II is geometrically similar to domain I but lineations have east-west trends (Figures 6a and 6d). Domain II also contains a complex network of conjugate melt filled brittle-ductile shear zones (Figure 9d). The conjugates are geometrically similar to a network of listric normal faults (Figure 9d). Northwest side down (sinistral) shears are the most numerous set, suggesting that the flow was dominated by a top to the west-northwest displacement similar to domain I. The average orientation of the synthetic (sinistral) and antithetic (dextral) shears was calculated and used to estimate the orientations of the finite strain axis in this area (Figure 6e). The geometry of the shears indicates that the shortening axis bisects a 110 angle and the foliation bisects the acute angle between the shears. Ductile conjugate shears should form at 90 angles according to the van Mises failure criterion [Twiss and Moores, 1992]. This suggests that after their formation the shears rotated toward the foliation due to progressive flattening. [34] In calculating the orientations of the finite strain axes, we have assumed that the shortening direction bisects the obtuse angle between the shears and that the intermediate strain axis lies parallel to the intersection of the shears, consistent with the methods described by Ramsay and Huber [1983]. Accordingly, the shortening direction is concluded to plunge moderately to the south and the extension axis is inferred to be horizontal and east-west trending (Figure 6e). Shortening and extension directions inferred from the conjugates are consistent with those

11 ANDRONICOS ET AL.: EXTENSION IN THE COAST PLUTONIC COMPLEX 7-11 Figure 8. (a) Map summarizing finite strain and kinematic pattern around the Kasiks sill complex. (b) Stereonet showing contours of all foliations from the Kasiks sill complex and Shames River mylonite zone. (c) Stereonet showing contours of all lineations from the Kasiks sill complex and Shames River mylonite zone. estimated from the orientation of mineral lineation and foliation within domains I and II (Figures 6b, 6d, and 6e). The orientations of these axes are also nearly identical to those determined by Davidson et al. [1994] for conjugate tensile fractures at the base of the Kasiks sill complex along the Skeena River Shames River Mylonite Zone [35] Domain III structurally overlies domains I and II and is dominated by the Shames River mylonite zone (Figures 6a and 6h) described by [Heah, 1991a]. Foliations within domain III are subparallel to those in the other two domains, but lineations plunge consistently toward the northeast (Figure 6f). Going east and toward structurally higher levels from domain II to domain III, small offset brittle faults and minor brittle-ductile shears crosscut the gneissic layering (Figures 6a, 6h, and 10a). Shears and faults display east side down normal sense displacement (Figures 6a, 6h, and 10c). A prominent dip slope occurs on the western side of the Exstew River valley (Figures 6a and 6h). The base of this dip slope coincides approximately with the base of the Shames River mylonite zone (Figure 6h). [36] The S 3 foliation within domain III is mylonitic, as opposed to gneissic, and it displays microstructures indicative of middle amphibolite to greenschist facies temper-

12 7-12 ANDRONICOS ET AL.: EXTENSION IN THE COAST PLUTONIC COMPLEX Figure 9. Outcrop photos of structures in eastern Kasiks sill complex in area of Figure 6, west of Shames River mylonite zone. (a) and (b) Overprinting relationships showing the superposition of S2 on S1. (c) Symmetrical boudins in amphibolite interlayered with sills in the eastern Kasiks sill complex. (d) Conjugate melt-filled ductile shears from domain II of Figure 6. atures during deformation (Figures 10b and 10c). For example, mylonitic fabrics in granodiorite orthogneiss display quartz microstructures such as ribbon-shaped grains, deformation bands, sutured grain boundaries and undulatory extinction all indicating that quartz was deforming plastically (Figure 10b) [Fitzgerald and Stünitz, 1993]. In contrast, adjacent feldspar grains are cut by brittle fractures and are partially replaced by retrograde white mica and epidote (Figure 10b). These textures suggest that the mylonite was deforming at upper greenschist to middle amphibolite facies metamorphic conditions [Fitzgerald and Stünitz, 1993; Passchier and Trouw, 1996]. This contrasts with the melt present deformation textures that dominate the rest of the Kasiks sill complex and its surrounding high-grade gneiss. However, some plagioclase grains have deformation bands, new grains and undulatory extinction indicative of plastic deformation prior to the development of the observed brittle microstructures. This implies that deformation proceeded from higher to lower temperatures during the evolution of the mylonite zone. [37] Kinematic indicators are ubiquitous throughout the mylonite zone and include c/s fabrics, c shear bands, sigma- and delta-type porphyroclasts, tension vein arrays, and asymmetrical boudinage. They consistently indicate east-northeast side down sense of displacement (Figures 6a, 6h, and 10c). Crosscutting relationships indicate that east side down displacement within the mylonite zone postdates high temperature deformation within the Kasiks sill complex (Figure 10a). [38] Intruding all three domains in the area of Figure 6 are steeply dipping pegmatite and aplite dikes. In domains I and II, these dikes are undeformed and northwest striking with variable east and west dips. They clearly postdate penetrative ductile deformation. In contrast, aplite and pegmatite dikes within domain III vary from undeformed to strongly deformed with mylonitic fabrics (Figure 10d). The variable crosscutting relationships and concentration of aplite into tension cavities and veins suggest the mylonite zone was deforming while the dikes were intruding. [39] The dikes in domain I and to a lesser extent those in domain II display a number of features that suggest that they intruded as transitional tensile fractures. These dikes are locally abundant enough to account for 20% longitudinal extension. Mutually crosscutting relationships show that east and west dipping dikes intruded contemporaneously. East dipping dikes show minor east side down displacements across the dike walls and west dipping dikes show minor west side down displacements. The average orientations of the conjugates was calculated and used to estimate the instantaneous strain field (Figure 6c). As the dikes intruded, the greatest compression axis was vertical, the intermediate strain axis was subhorizontal and northwest trending, and the stretching axis (or least compression axis) was horizontal and northeast-southwest trending (Figure 6c) Shames River Fault Zone [40] On the eastern side of the area shown in Figure 6a, the high angle brittle Shames River Fault zone [Heah, 1991a, 1991b] cuts the Shames River Mylonite (Figure 6h). This fault zone represents the contact of high-grade gneiss and midcrustal plutons with low-grade volcanic and sedimentary rocks of the Stikine Terrane [Heah, 1991a, 1991b]. Fault gouge from minor faults associated with the Shames

13 ANDRONICOS ET AL.: EXTENSION IN THE COAST PLUTONIC COMPLEX 7-13 Figure 10. Outcrop photos of structures related to Shames River mylonite. (a) Brittle-ductile shears cutting gneisses near base of Shames River mylonite. (b) Photomicrograph of quartz and feldspar microstructures from granodiorite orthogneiss in Shames River mylonite. (c) Feldspar clast with asymmetry indicting northeast directed shear in Shames River mylonite. (d) The 52 Ma late kinematic dikes in mylonite zone. River fault typically contains the greenschist facies mineral assemblage actinolite-epidote-chlorite-calcite-quartz ± hornblende. Aggregates of chlorite and actinolite help define the slickensides on minor fault planes. The occurrence of these mineral assemblages and textures suggests that the fault zone was deforming during greenschist facies metamorphic conditions, between 300 C and 500 C [Apted and Liou, 1983]. [41] Slip directions on minor normal faults associated with the Shames river fault zone were used to estimate the greatest compression (P) axis and least compression (T) axis using the method of Marret and Allmendinger [1990]. The orientations of P axes were assumed to approximate the shortening direction, whereas the T axes were assumed to approximate the stretching axis [Marret and Allmendinger, 1990]. This analysis suggests subvertical shortening and northeast directed horizontal stretching, similar to that determined from the analysis of dikes in domains I and II (Figure 6g). [42] To summarize the deformation history of the area of Figure 6, early S 1 fabrics (equal to S 2 at Toon Lake) are rarely preserved but are similar in orientation to fabrics related to dextral transpressive shearing found south of the Skeena river and between the Quottoon and Alistair Lake plutons and at Toon Lake (Figure 1). This fabric is overprinted by S 2 (equal to S 3 at Toon Lake), which dips north and has an east-west strike in domains I and II. Kinematic indicators in domains I and II are consistent with top to the west-northwest normal shearing. The structural analysis shows these structures formed during moderately south plunging shortening and east-west subhorizontal extension. Fabrics related to the Shames River mylonite zone overprint these structures. Kinematic indicators indicate a consistent northeast side down sense of displacement within the Shames River mylonite zone. Dike orientations suggest vertical shortening and northeast trending stretching during mylonitization. Microstructures indicate the mylonite deformed during decreasing

14 7-14 ANDRONICOS ET AL.: EXTENSION IN THE COAST PLUTONIC COMPLEX

15 ANDRONICOS ET AL.: EXTENSION IN THE COAST PLUTONIC COMPLEX 7-15 Figure 12. Map showing locations of 40 Ar/ 39 Ar and U/Pb geochronologic samples. temperature conditions. The mylonite is cut by the brittle Shames River fault, which formed during vertical shortening and east-northeast directed stretching. It is most likely that the Shames River fault and Shames River mylonite represent a continuum of deformation. 4. Geochronology [43] Extant 40 Ar/ 39 Ar biotite and hornblende cooling ages were combined with newly acquired U/Pb zircon crystallization ages and sphene cooling ages to constrain the timing of the deformational events described above. Cooling ages are compared with deformational microstructures and metamorphic data to constrain the relationship of the deformational events to the cooling history of the region U/Pb Zircon and Sphene Dating [44] U/Pb ages are reported in Figure 11. Their locations are shown on Figure 12, and the isotopic data are shown in Table 1. Ages were obtained using standard isotope dilution methods. [45] Sample is of the Alistair Lake pluton and was collected from the central part of the pluton in the southeast Figure 11. (opposite) Concordia diagrams for U/Pb zircon and sphene samples discussed in text. (a) Concordia diagram for sample from central part of the Alistair Lake pluton. (b) Concordia diagram for sample from asymmetric meltfilled boudin collected at Exstew along Highway 16 between Terrace and Prince Rupert, British Columbia. (c) Concordia diagram for sample of Kasiks pluton collected at Railroad tunnel along Highway 16, between Terrace and Prince Rupert, British Columbia. (d) Concordia diagram for sample of Kasiks sill complex collected from roof of complex near the Shames River mylonite. (e) Concordia diagram for pegmatite dike collected from same locality as Exchamsiks sample. Also from the dike swarm depicted in Figure 6c. (f) Concordia diagram for late kinematic dike collected from Shames River Mylonite along Highway 16 road cuts east of Exstew River. (g) Concordia diagram for sample from base of Ponder pluton at Luz del Sol Ridge. (h) Sample of leucogranite pluton collected from area of Figure 5a. See text for discussion of dates.

16 7-16 ANDRONICOS ET AL.: EXTENSION IN THE COAST PLUTONIC COMPLEX Table 1. U-Pb Isotopic Data and Ages a Grain Characteristics Apparent Ages, Ma Size Number of Grains Weight, mg Pb c, pg U, ppm 206 Pbm 204 Pb 206 Pb 208 Pb 206 Pbb 238 U 207 Pbb 235 U 206 Pbb 207 Pb C-68 ZE ± ± ± 9 ZCt ± ± ± 11 ZAa ± ± ± 20 ZAa ± ± ± 30 ZAa ± ± ± 23 ZAa ± ± ± 17 ZAa ± ± ± 23 EXSTEW ZG ± ± ± 8 ZD ± ± ± 12 ZD ± ± ± 21 ZC ± ± ± 17 ZC ± ± ± 54 ZA ± ± ± 20 SB ± ± ± ZC ± ± ± 15 ZCt ± ± ± 15 ZB ± ± ± 14 ZAa ± ± ± 32 ZAa ± ± ± 27 ZAa ± ± ± 26 ZAa ± ± ± 30 ZAa ± ± ± ZE ± ± ± 13 ZC ± ± ± 19 ZCt ± ± ± 10 ZB ± ± ± 15 ZAa ± ± ± 14 ZAa ± ± ± 17 ZAa ± ± ± 32 ZAa ± ± ± 16 ZAa ± ± ± ZE ± ± ± 10 ZCt ± ± ± 18 ZC ± ± ± 16 ZA ± ± ± 15 KASIKS ZDa ± ± ± 24 ZCa ± ± ± 86 ZDa ± ± ± 16 ZDa ± ± ± 36 ZDa ± ± ± 33 SA ± ± ± 180 SA ± ± ± 170 EXCHAMSIKS ZD ± ± ± 12 ZC ± ± ± 17 ZBa ± ± ± 49 ZA ± ± ± 11 ZAa ± ± ± 37 ZAa ± ± ± 55 SA ± ± ± 190 SA ± ± ± 150 PEG DIKE ZC ± ± ± 89 ZB ± ± ± 14

17 ANDRONICOS ET AL.: EXTENSION IN THE COAST PLUTONIC COMPLEX 7-17 Table 1. (Continued) Grain Characteristics Apparent Ages, Ma Size Number of Grains Weight, mg Pb c, pg U, ppm 206 Pbm 204 Pb 206 Pb 208 Pb 206 Pbb 238 U 207 Pbb 235 U 206 Pbb 207 Pb ZA ± ± ± 9 a Grain size: A, mm; B, mm; C, mm; D, mm; E, mm; F, 63 80mm; G, 45 63mm; a, abraded; t, tan/cloudy (slightly metamict) grain; Pb c, total common Pb in picograms. The 206 Pb m / 204 Pb is measured ratio, uncorrected for blank, spike, fractionation, or initial Pb. The 206 Pb/ 208 Pb 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. Decay constants: 235 U = , 238 U = , 238 U/ 235 U = All uncertainties are at the 95% confidence level. Pb blank generally ranged from 2 to 10 pg. U blank was <1 pg. Isotope ratios are corrected for fractionation of 0.14 ± 0.10 %/amu for Pb and 0.20 ± 0.40 for UO 2. Initial Pb composition is interpreted from Stacey and Kramers [1975], with uncertainties of 1.0 for 206 Pb/ 204 Pb, 0.3 for 207 Pb/ 204 Pb, and 2.0 for 206 Pb/ 208 Pb. All analyses conducted using conventional isotope dilution and thermal ionization mass spectrometry, as described by Gehrels [2000]. b Radiogenic Pb. part of the study area. Hollister and Andronicos [2000] described features at the base of the pluton, which indicated dextral transpressive shearing had deformed the base of the pluton along its western side (fabrics similar in geometry and kinematics to Toon Lake S 1 ). In contrast, in the area where the dated sample comes from, the pluton was strongly deformed with gently dipping gneissic fabrics. High-angle ductile shear bands that indicate north side down sense of displacement cut this fabric. The lack of leucosome concentrated in the shear bands and the gneissic fabric suggest much of the deformation postdated intrusion of the granite. Three multigrain zircon fractions and five single grains were analyzed. These analyses define an apparent discordia with a lower intercept of 86 ± 3 Ma and an upper intercept of 327 ± 39 Ma (MSWD = 1.4, Figure 11a). The lower intercept is interpreted as the crystallization age, whereas the upper intercept is the average age of inherited components. We interpret the age of Alistair Lake pluton to be the maximum age of deformation described within this paper since it is affected by all the phases of deformation. [46] A sample of leucogranite filling an asymmetric boudin neck was collected from just west of the Exstew river valley at the base of the Kasiks sill (Figure 11b). The asymmetry of the boudin indicates top to the northwest directed shearing. Five multigrain fractions of zircon, two single zircon grains, and one sphene fraction were analyzed. All of the zircon analyses are apparently concordant, with an interpreted crystallization age of 53.2 ± 1.2 Ma. The single sphene fraction is 49 ± 2 Ma. The zircon date indicates that the west-northwest directed shearing was ongoing at 53.2 Ma in the area of Figure 6. [47] A sample of the Kasiks sill complex was collected from the main body along the Skeena River (Figure 11c). Three single zircon grains and two multigrain fractions of sphene were analyzed. The zircon grains are concordant at 53.4 ± 1.3 Ma, which we interpret as the crystallization age. The sphene analyses are concordant at 52 ± 2 Ma. The zircon date shows that the main body of the Kasiks sill intruded at 53.4 Ma. [48] An additional sample of the Kasiks sill complex was collected from within the area of Figure 6. This sill was affected by top to the northwest normal shearing. Three multigrain fractions of zircon, three abraded single zircon grains, and two sphene fractions were analyzed. All of the zircon analyses are concordant at 52.5 ± 1.5 Ma, and the sphene fractions are concordant at 50.8 ± 1.5 Ma (Figure 11d). The zircon date, combined with the date from the Skeena River, suggests that most of the Kasiks sill complex was constructed at 53 Ma. [49] A tabular, undeformed pegmatite dike that crosscuts the sill at Exchamsiks was collected to determine when northwest directed shearing ended (Figure 11e). This dike is also part of the swarm of dikes used to estimate the orientations of the instantaneous strain in domain II of Figure 6. Only a few zircon grains were recovered from this sample. Three single zircon grains were analyzed, two of which are apparently concordant at 52 ± 2 Ma. The third grain is presumably discordant due to inheritance of slightly older material. This age suggests that northwest directed ductile shearing ended by 52 Ma in the area of Figure 6 and that the region was undergoing subvertical shortening and northeast directed horizontal stretching. [50] A sample of a late kinematic dike (98-16) was collected from within the Shames River mylonite zone with two goals in mind. The first was to constrain whether the dikes within the Shames River mylonite were coeval with those that intruded the underlying sill complex. The second was to constrain the timing of the last increments of deformation within the mylonite zone. Four multigrain fractions and five abraded single grains were analyzed. All of the analyses are discordant, and they apparently define a discordia line with a lower intercept of 52 ± 4 Ma and an upper intercept of 191 ± 19 Ma (MSWD = 0.3) (Figure 11f ). The lower intercept is interpreted to be the crystallization age and the upper intercept the average age of inherited components. This suggests that these dikes are coeval with those in the footwall of the mylonite zone and that ductile deformation within the mylonite was ongoing at 52 Ma. [51] Sample C-68 is from a sill that intrudes a subhorizontal shear zone with top to the east sense of shear along the western side of Ponder Pluton. The sill has magmatic fabrics concordant with mylonitic foliations within the host gneisses suggesting it intruded during shearing. Seven analyses were conducted: two on multigrain fractions and five on single abraded grains. The analyses define an apparent discordia, with an upper intercept of 224 ± 27 Ma and a lower intercept of 52 ± 4 (MSWD = 1.0, Figure 11g). Discordance is interpreted to result from crystallization at 52 Ma with inheritance of