Tectonic overview of the CPC

The tectonics of the northeast Pacific has been shaped principally by the interactions of oceanic plates of the Pacific basin with the leading edge of North America (Engebretsen et al., 1985; Lonsdale, 1988; Stock and Molnar, 1988). Subduction drove major deformation events and led to the construction of Cordilleran batholiths along the length of the North American plate margin. Here we briefly describe the changes in plate kinematics as currently understood and how these relate to regional events within the proposed study area. The Canadian Cordillera, within the northeast Pacific, has been the site of major orogenic events since the mid-Paleozoic. Crustal thickening and subsequent batholith emplacement led to the generation of two major Mesozoic plutonic/metamorphic welts within the Cordillera, the Omineca belt and the Coast Plutonic Complex (Monger et al., 1982). The Coast Plutonic Complex (CPC) is built on exotic island arc fragments, oceanic plateaus and continental margin assemblages accreted between the Triassic and the Cretaceous (Monger et al, 1982). These accreted terranes consist of two major tectonostratigraphic packages, the Intermontane Superterrane to the east of the CPC, and the Insular Superterrane to the west. The Mesozoic batholiths of the CPC stitch and overprint the boundaries of these packages.

 

Magmatic Belts

The proposed study area is within the Coast Plutonic Complex (Figure 1 and 2), which records voluminous plutonism from the Jurassic to the Tertiary. Within the proposed study area, plutons of the CPC form four belts that parallel the general northwest-trending structural grain of the orogen (e.g., Armstrong, 1988; van der Heyden, 1992; Gehrels and Boghossian, 2000)(Figure 2). From west to east these belts are: (1) 160-100 Ma diorite to granite plutons that intrude juvenile arc-type crust of the Alexander terrane, (2) 100-90 Ma epidote-bearing tonalite plutons that were emplaced into the suture zone between the Alexander terrane and an inboard continental margin assemblage (Nisling terrane), (3) 70-57 Ma tonalite bodies, generally highly elongate, that were emplaced into and are syntectonic with the Coast shear zone (including the Great Tonalite Sill), and (4) Eocene (57-50 Ma) tonalite to granite bodies of various sizes and emplacement depths that were intruded into this continental margin assemblage and into the Stikine terrane to the east (Figure 2).

Plate kinematics during the formation of the Coast Plutonic Complex are poorly constrained owing to the small percentage of Mesozoic oceanic crust preserved in the Pacific Ocean basin (Engebretsen et al., 1985) and to probable absolute motion of the Pacific hotspots during Late Cretaceous and Tertiary time (e.g. Norton, 1995; Stock and Molnar, 1988; Raymond et al., 2000; Tarduno et al., 2001). In spite of these uncertainties, it is clear that the Coast Plutonic Complex formed during subduction of the Kula and Farallon plates along the Cordilleran margin. Following is an overview of relative plate motions along the continental margin, which is based on analyses of Engebretson et al. (1985), Stock and Molnar (1988), and Lonsdale (1988). It should be noted, however, that recent documentation of southward motion of the Hawaiian hot spots relative to Indo-Atlantic hot spots (Raymond et al., 2000; Tarduno et al., 2001) will require significant revision of these relative plate motions.

Prior to mid-Cretaceous time, subduction between North America and Farallon plates appears to have been dominated by contraction and sinistral transpression (Engebretsen et al., 1985). Geological studies within the North American Cordillera are consistent with this view as major shortening events are found along the length of the Cordillera and sinistral transpression has been recognized along much of the Cordillera during this time (e.g. Chardon et al., 1999; Evenchick, 1991 and 2001; Monger et al., 1994). During the construction of the 90 to 100 Ma plutons, plate convergence was largely orthogonal, with a small sinistral component (Engebretsen et al., 1985). Following construction of 90 to 100 Ma plutons plate convergence rates appear to have accelerated and dextral transpressive convergence dominated. During construction of 70 to 57 Ma plutons convergence may have been as fast as 200 mm/yr (Engebretsen et al., 1985) during the northward motion of the Kula Plate. Stock and Molnar (1988) concluded that convergence during this time period was slower, varying between ~140 and 110 mm/yr. Both reconstructions require a significant dextral transcurrent component to convergence. During emplacement of the 57 to 50 Ma plutons plate motions appear to have become even more oblique, with a large dextral component (Lonsdale, 1988). This change in motion appears to coincide with the onset of largescale dextral transtensional deformation throughout the Canadian Cordillera (Struik, 1993; Andronicos et al, in press). Calc-alkaline magmatism ended between 50 and 45 Ma (Armstrong, 1988), and by 45 Ma the Queen Charlotte transform plate boundary was born.

 

The Late Cretaceous-Early Tertiary Magmatic Front

The most prominent feature within the central and northern CPC is the early Tertiary magmatic front that divides the CPC and marks the western edge of ~70 to 45 Ma plutons (Figure 2). Armstrong (1988) first pointed out that plutons with these ages occur exclusively east of a sharp magmatic front, but have a more diffuse magmatic rear (Figure 2). Results of the geochronologic portion of the ACCRETE project confirmed that this magmatic front exists between 55 and 54 o N and showed that in this region most of these plutons intruded in the time interval between 70 and 50 Ma (Figure 6b). This flare-up is also associated with a change in e Nd values suggesting significantly more crustal melting then during other magmatic episodes within the Coast Plutonic Complex (Figure 6). Fundamental to understanding the generation of the Late Cretaceous and early Tertiary portion of the CPC is understanding what tectonic processes led to the formation of the magmatic front and how the front relates to geochemical trends within the CPC.

 

Coast Shear Zone

The early Tertiary magmatic front generally coincides with the western edge of Coast shear zone, a ductile shear zone that divides the CPC along its axis for more than 1200 km (Figures 1 and 2). The Coast shear zone had a complex deformation history including: (1) early dextral transpressive displacements between 85 and 60 Ma; (2) northeast-side-up reverse motion between 65 and 57 Ma; and (3) normal northeast-sidedown motion between 57 and 48 Ma (Klepeis et al., 1999; Andronicos et al., 1999; Rusmore et al., 2001). A string of 70-55 Ma syntectonic tonalite plutons intruded the shear zone, but are absent to the southwest (Brew and Ford, 1978; Stowell and Hopper, 1990; Gehrels et al., 1991; McClelland et al . , 1992; Ingram and Hutton, 1994; Klepeis et al . , 1998; Thomas and Sinha, 1999; Andronicos, et al., 1999; McClelland et al., 2000; Rusmore, et al., 2001). The spatial coincidence of the shear zone and the magmatic front is clear north of 52 o N, but the extent of this relation to the south and its role in the evolution of the CPC remain uncertain.

 

Central Gneiss Complex

From the Alaska British Columbia border to south of Douglas Channel, early Tertiary and Late Cretaceous plutons are hosted by a high-grade migmatite-gneiss terrane called the Central Gneiss Complex (CGC; Figure 2) (Hutchison, 1982). The CGC consists of metavolcanic, metaplutonic, and metasedimentary rocks that were metamorphosed to the upper amphibolite and lower granulite facies (Hollister, 1975; Hollister and Crawford, 1990; Hollister and Andronicos, 2000). As such these rocks provide the most direct evidence for the types of lithologies that we expect to see in the middle and lower crust within the study area providing important constraints on the interpretation of the seismic and gravity data. Additionally, study of the burial and exhumation history of these rocks provides some of the best constraints on how crustal thickness changes with time.

The general character of the package suggests derivation from lower Paleozoic continental margin strata (Nisling terrane) and from Devonian through Jurassic arc-type rocks of the Stikine terrane, which is part of the Intermontane Superterrane (Hill et al, 1985; Gareau and Woodsworth, 2000; Douglas, 1986; Rusmore et al., 2000; Gehrels and Boghossian, 2000; Boghossian and Gehrels, 2000). The Stikine terrane is a juvenile arc-type terrane to the east of the CPC that dominates the interior of British Columbia .

The largest volume of plutons north of Douglas Channel in the ACCRETE study area was confined to the relatively short time interval from 65 Ma to 50 Ma (Figure 6b). The entire CGC and associated plutons cooled rapidly in the Eocene. Sphene U/Pb and 40 Ar/ 39 Ar hornblende ages show these rocks cooled to ~550 o C by 52 - 51 Ma, and Ar/Ar dates on biotite show that the rocks reached ~250 o C by ~49 Ma (Andronicos et al., in press). Enough samples have been dated to show that the cooling was nearly synchronous over the 40 km-wide block, and that the cooling was nearly synchronous with motion on a ductile normal shear zone that separates the CGC from Stikine Terrane (Heah, 1990, 1991; Andronicos et al., in press).

Between Douglas Channel and Bella Coola (Figure 2 and Figure 7), the CGC terminates along the axis of the orogen and there is marked decrease in the volume of early Tertiary and Late Cretaceous plutons. In the Bella Coola area, greenschist facies sedimentary and volcanic rocks of the Stikine terrane host scattered plutons of Late Cretaceous to Tertiary age. Within the proposed northern transect, north of Douglas Channel, Eocene plutons have intruded as concordant sheets contemporaneously with 15-20 km of exhumation (Hollister, 1982; Hollister and Andronicos, 2000; Andronicos et al., in press); within the proposed southern transect, near Bella Coola, Eocene plutons intruded into the upper half of the crust without a large amount of exhumation (Baer, 1973; Rusmore et al., 2000 and 2001). The close timing of the extensional deformation and the rapid cooling of the CGC north of Douglas Channel suggest that large-scale crustal extension facilitated the denudation and cooling of the CGC (Gehrels and McClelland, 1988; Heah, 1990, 1991; Andronicos et al., in press). A critical question is whether or not similar extensional shear zones continue to the south of the Douglas Channel area. If the shear zones die out prior to reaching the Burke/Dean Channel area, then this may explain the termination of the CGC and the decrease in volume of exposed Eocene plutons, which may not have been exhumed in this area (Figure 7). On the other hand, the ductile shear zones might project under the Burke/Dean Channel transect. If so, they should be recognizable in the seismic data, and brittle structures exposed at the surface should be compatible with extensional deformation along an underlying detachment fault. It is essential to distinguish between these alternatives and to define the relation between the generation of the magmatic front and crustal extension. The amount of extension since the Miocene was relatively minor (Rusmore et al, 2000; Farley et al., 2001) compared to the Eocene extension event(s) recognized in the ACCRETE study area (Andronicos, et al., in press). No subduction-related tectonism or magmatism has occurred since at least 45 Ma under either proposed transect. This contrasts sharply with the situation beginning about 100 km south of the southern transect and extending through the northern Cascades, where the lower crust and upper mantle are now under the influence of the present-day Cascadia subduction zone. This subduction makes it impossible to correlate lower crustal features imaged seismically with older events reflected in the emplacement of the batholiths now exposed at the surface in the northern Cascades or the southern CPC.

 

Post Arc Magmatism

The CPC has been intruded by a widespread basaltic dikes (Hutchison, 1980; Baer, 1973). These dikes, although not voluminous, occur throughout the study area and provide an important sampling of the post-arc lithosphere. Additionally, widespread volcanic belts such as the Anahim volcanic field lie adjacent to the east of the study area. These rocks and the xenoliths that they host are being studied by one of our Canadian Collaborators, J. Kelly Russell of the University of British Columbia . We will work with Kelly to integrate our results with his. Unfortunately, to our knowledge, no xenolith-bearing mafic rocks have been found within the CPC itself.