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The following abstract has been sent to the SNORCLE Workshop, scheduled for March 7-9, 1997, University of Calgary.

For additional information see links to SNORCLE and LITHOPROBE

Geological data cannot rule out large scale (>1000 km) transcurrent motion across the Coast orogen.

The Coast shear zone (Fig. 1) forms a crustal-scale boundary between segments of the Coast Ranges that record different structural, metamorphic, and intrusive histories. It is a candidate deep crustal structure for large scale (>1000 km) transcurrent strike-slip motion at the time of postulated northwards translation of the Insular superterrane between 85 and 60 Ma. The range of published U/Pb pluton dates (83-48 ma) within the central gneiss complex (CGC), which is located between the Coast shear zone and the Stikine terrane (Fig. 2a), overlaps the time of the postulated transcurrent motion. We therefore include the entire CGC as a region where the orogen-parallel transcurrent motion could occur, on horizontal as well as vertical structures.

Postulated plate motions between 85 and 60 Ma are transpressive. Because the rocks of the CGC were at mid to lower crustal depths during the late Cretaceous and early Tertiary, and were intruded by plutons during that interval, this region deformed ductily during the transpression. The interplay of transpression during widely distributed ductile flow and periodic intrusion of plutons produced a complex strain field that is highly variable in space and time. Hence, recognizing large scale orogen parallel displacement across such a gooey mess is difficult.

One goal of ACCRETE is to focus the collective expertise of metamorphic and structural geologists on this problem. Its resolution is extremely important: if we can identify how the lower crust behaves during transpressive deformation, we will better understand how crustal scale strike-slip faults work.

Our work to date suggests the following:

1) Near vertical, high-strain zones occur along the Coast shear zone for >800 km. Two phases of deformation are recognized. The earliest deforms 65 Ma plutons and produced dominantly coaxial shear with northeast-southwest flattening strains. The second deformation, with a date of about 55 Ma, produced steep to sub-vertical fabrics and a kilometer-scale dextral deflection of older fabrics along the west side of the CGC. The east-side-down component of the oblique-dextral shear couple model is probably due to crustal relaxation following cessation of compressive deformation. The minimum dextral component, which is indicated by deformation in the wall rock of the shear zone only, is on the order of 10s of km.

2) Further east, within the CGC, pre 58 Ma (a date of the Quottoon pluton, Fig. 2a) deformation involved generally top to the south thrusting (Fig. 2b). This thrusting appears to have occurred from at least 83 Ma until perhaps 58 Ma. Fabrics formed by this thrusting predate the coaxial strain mentioned above, which may be manifest in folds of the thrust fabric with upright axial planes. A high T/P metamorphic overprint on moderate pressure rocks occurred late in the thrusting history and was probably related to exhumation of the hot, thick thrust pile.

A 3 km thick dextral transcurrent ductile shear zone with a dip-slip component has been identified at the eastern margin of the Quottoon pluton (Fig. 2a). The north-south extent of the shear zone is currently unknown; it has a minimum length of 25 km. It trends to the north-northeast, and mainly predates intrusion of the Quottoon pluton, although it probably overlapped final crystallization. Transport direction inferred for the shear zone from c/s intersections plunges 2° to 022°. In contrast, the mean stretching lineation plunges 31° to 340°. This apparent contrast in transport directions is likely to be due to partitioning of transpressive strain, and/or rotation of mineral lineations. In the southeastern mapped portion of the shear zone, lineations plunge about 40° NNW and fabrics dip steeply to the east and west. Dextral shearing was synchronous with the formation of sheath folds with fold axes parallel to the mineral stretching lineations in this area. Refolded folds within this zone suggest that movement on this shear zone largely post-dates thrusting, although formation of sheath folds appears to have been contemporaneous with a portion of the thrusting. Thus, transcurrent motions were partitioned both horizontally and vertically within the shear zone and adjacent areas. The 3 km width of the shear zone and the intensity of fabrics developed within the shear zone suggest a minimum strike-slip displacement on the order of tens of kilometers.

3) Along the east side of the CGC is the Shames mylonite zone mapped by Heah (1991); it is over 4 km thick (Fig. 2b) and may be traceable north to northeasterly along the east side of the Coast Mountains up to Portland Inlet. Kinematic indicators show down-dip motion, and we interpret the shear zone as a major extensional fault. It is mainly cut by, or underlies, a belt of 50 Ma plutons. The lower contact of the detachment zone dips moderately east and cuts the top to the south thrust fabric at a low angle. Although mostly ductile, this shear zone formed at lower temperature than the earlier thrust deformation, based on microstructures. Its activity is constrained to be before 48 Ma and after 65 Ma. This overlaps the time interval of rapid cooling of the CGC.

This paper is a progress report on the geologic component of a coordinated attempt to place limits on estimates of where, when, and how much orogen parallel motion occurred within the Coast orogen. At present, the geologic data are highly suggestive that a large amount of strike-slip displacement could have occurred across the CGC during the time interval postulated for such displacement by paleomagnetic arguments. We emphasize (1) the low angle ductile thrusting which may represent lower crustal transfer of strike-slip motion across the orogen, (2) the plutons that intruded during deformation which must have produced a heterogeneous stress field leading to complex strain fields and reorientations of structures including lineations, and (3) the Shames detachment that obliterated earlier kinematic indicators of strike-slip or thrust displacements.

The dramatic contrast in pluton and cooling ages across its western boundary and the contrast of structural history between the CGC and adjacent blocks require major faulting along its margins and emphasize the importance of the CGC in the late Cretaceous to early Tertiary history of western North America.

See Abstract: Evidence for late (Miocene?) extension across the Coast Mountains

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