Search

This paper documents the newly recognized southern continuation of the early Tertiary Coast shear zone, extending its known length by ∼350 km to more than 1200 km. Three sites along the shear zone in British Columbia, Douglas Channel, Bella Coola, and Machmell River, have similar histories during the period ca. 65–55 Ma. The shear zone is 2–11 km thick and is defined by well-developed mylonite zones that strike northwest and dip steeply northeast. Motion on the shear zone was predominantly reverse, with the northeast side up. Synkinematic plutons are common in the shear zone. Lower plate rocks, high-grade gneiss derived from an ancient continental margin assemblage, show little evidence of the extensive deformation and plutonism in the shear zone. North of Bella Coola, high- grade gneiss forms the upper plate, but to the south weakly metamorphosed rocks of Stikinia compose the upper plate. Geochronologic data show that the shear zone was active between ca. 60 and 55 Ma at Douglas Channel, ca. 62 and 56 Ma at Bella Coola, and after 66 and before 56 Ma near the Machmell River. These features match those of the Coast shear zone in southeast Alaska and adjacent British Columbia. Together the shear zones formed a continental-scale reverse ductile fault in Paleocene time. Plate reconstructions show an oblique component to the dominantly dextral transcurrent Paleocene margin. Partitioning of this motion across the continental margin produced a regional system of strike-slip faults and contraction on the Coast shear zone.

New structural, paleomagnetic, and apatite (U-Th)/He results from the continental margin inboard of the Queen Charlotte fault (~54°N) delineate patterns of brittle faulting linked to transform development since ~50 Ma. In the core of the orogen, ~250 km from the transform, north striking, dip-slip brittle faults and vertical axis rotation of large crustal domains occurred after ~50 Ma and before intrusion of mafic dikes at 20 Ma. By 20 Ma, dextral faulting was active in the core of the orogen, but extension had migrated toward the transform, continuing there until

[1] New geologic data from the Central Gneiss Complex along Douglas Channel help delineate the burial and exhumation of the Late Cretaceous to Eocene Coast Mountains magmatic arc. Arc plutonism was on going between ∼90 and 60 Ma, replaced by formation of dikes to 52 Ma. Supracrustal rocks were buried to midcrustal levels by 90 Ma, and garnet and kyanite grew from ∼90 through 70 Ma. Subsequent exhumation was nearly isothermal and took place in two stages. The first occurred from ∼70 to 59 Ma as the arc contracted obliquely and sillimanite replaced kyanite. Exhumation was slow (∼0.5 mm/yr), probably accomplished by erosion aided by coaxial crustal thinning. Exhumation rates about doubled in the second stage of exhumation, after 59 Ma and before ∼52 Ma. Penetrative deformation ended prior to intrusion of the Quottoon pluton at 59 Ma and ∼6.5 kbar. Production of cordierite rims on garnet at 4.5 kbar and ∼700°C signaled the end of near-isothermal decompression prior to 52 Ma. Rapid cooling (100°C/106 years) of the Central Gneiss Complex followed cordierite growth; temperatures dropped to ∼250° by ∼48 Ma. The increase in exhumation rate and the subsequent rapid cooling are attributed to excision of >6 km of crust on a detachment system on the northeastern side of the Central Gneiss Complex. Comparison to other parts of the Coast Mountains arc, the Sierra Nevada, and Fiordland, New Zealand, shows that the amount and tempo of exhumation vary greatly within and between arcs, suggesting that the processes accommodating exhumation vary significantly.

Structural and stratigraphic data from the eastern Coast Mountains, British Columbia, point to the presence of a Late Cretaceous thrust belt on the western margin of Stikinia. In the Bella Coola region, a fragment of thisbelt is preserved as the Sheemahant shear zone and its lower plate strata, the Early Cretaceous Monarch volcanics and Taylor Creek Group. The Sheemahant shear zone strikes northwest (∼300), dips moderately (∼55°) southwest, and verges to the northeast. Tonalitic protomylonites and mylonites within the shear zone constitute the Mt. Daunt orthogneiss. Fabrics within the orthogneiss and metamorphic patterns suggest that thrusting occurred during or soon after emplacement of the Mt. Daunt orthogneiss. U-Pb dating yields a crystallization age of 91 ′ 3 Ma for the orthogneiss, suggesting that the Sheemahant shear zone was active in Late Cretaceous time. After thrusting, the upper plate of the Sheemahant shear zone was cut by the early Tertiary Coast shear zone and intruded by the Sheemahant pluton. The Sheemahant pluton has a biotite cooling age of 54 Ma, placing a younger limit on the age of the Sheemahant shear zone. Reconstruction of the upper plate of the shear zone suggests that amphibolite facies gneiss of the Burke Channel assemblage composed the highest parts of the upper plate. This assemblage was metamorphosed and deformed prior to 82 Ma and appears to belong to a suite of Precambrian to Paleozoic volcanic-rich continental margin assemblages present in the core of the central and northern Coast Mountains. The Sheemahant shear zone is probably coeval with and kinematically linked to the eastern Mt. Waddington thrust belt and coeval thrusts near Whitesail Lake. The Monarch volcanics and Taylor Creek Group are correlated with Lower Cretaceous units in these areas and are interpreted as a coherent volcanic arc built on the western edge of Stikinia. Continuity of the thrust belt and arc strengthens the view that a northeast-vergent thrust belt formed the western margin of Stikinia in mid-Cretaceous time. This conclusion reinforces the interpretation that middle to Late Cretaceous arc magmatism in the Coast Mountains was coeval with regionally extensive contractional deformation. Existence of a coherent thrust belt along the western margin of Stikinia is difficult to reconcile with the ∼3000 km of northward transport of western British Columbia suggested by paleomagnetic data. If this interpretation of the paleomagnetic data is correct, either the thrust belt was not continuous, or parts of Stikinia had different transport histories.

Alpine glaciers are efficient agents of erosion and capable of significantly modifying topography. Despite recent advances in theoretical and field studies that quantify glacial erosion processes, few studies have documented glacial erosion rates over long (>10 6 yr) or large (more than tens of kilometers) scales. We use apatite (U-Th)/He (AHe) and apatite fission track (AFT) cooling ages to address the late Miocene to Holocene erosion history across two 60-km-long transects of the heavily glaciated southern Coast Mountains, British Columbia. Observed AHe cooling ages from equal elevation samples range between 1.5 and 8 Ma and suggest that thick alpine glaciers resulted in a 16 km shift of the highest point in the topography in the past 1.5-4.0 m.y. We evaluated temporal and spatial variations in erosion rates using a three-dimensional thermal-kinematic model that predicted AHe and AFT ages at the surface for different erosion histories. Comparison of model predicted and observed cooling ages suggests an increase in erosion rates of as much as 300% over the past 1.5-7 m.y., coincident with the onset of glaciation of this range.

Two small lower Mesozoic terranes, the Bridge River and Cadwallader, lie along the southwestern margin of the Intermontane superterrane and represent fragments of a volcanic arc and marginal basin that bordered North America in the early Mesozoic. During Middle Jurassic time, these terranes were juxtaposed and deformed. This event was synchronous with deformation in northern and central British Columbia, and it probably records accretion of the Cadwallader and Stikine volcanic arcs against Quesnellia during closure of the Bridge River-Cache Creek ocean basin.