Margaret E. Rusmore

U-Th-Pb geochronology of the Coast Mountains batholith in north-coastal British Columbia: Constraints on age and tectonic evolution

Previously published and new U-Pb geochronologic analyses provide 313 zircon and 59 titanite ages that constrain the igneous and cooling history of the Coast Mountains batholith in north-coastal British Columbia. First-order findings are as follows: (1) This segment of the batholith consists of three portions: a western magmatic belt (emplaced into the outboard Alexander and Wrangellia terranes) that was active 177–162 Ma, 157–142 Ma, and 118–100 Ma; an eastern belt (emplaced into the inboard Stikine and Yukon-Tanana terranes) that was active ca. 180–110 Ma; and a 100–50 Ma belt that was emplaced across much of the orogen during and following mid-Cretaceous juxtaposition of outboard and inboard terranes. (2) Magmatism migrated eastward from 120 to 80 (or 60) Ma at a rate of 2.0–2.7 km/Ma, a rate similar to that recorded by the Sierra Nevada batholith. (3) Magmatic flux was quite variable through time, with high (>35–50 km 3 /Ma per km strike length) flux at 160–140 Ma, 120–78 Ma, and 55–48 Ma, and magmatic lulls at 140–120 Ma and 78–55 Ma. (4) High U/Th values record widespread growth (and/or recrystallization) of metamorphic zircon at 88–76 Ma and 62–52 Ma. (5) U-Pb ages of titanite record rapid cooling of axial portions of the batholith at ca. 55–48 Ma in response to east-side-down motion on regional extensional structures. (6) The magmatic history of this portion of the Coast Mountains batholith is consistent with a tectonic model involving formation of a Late Jurassic–earliest Cretaceous magmatic arc along the northern Cordilleran margin; duplication of this arc system in Early Cretaceous time by >800 km (perhaps 1000–1200 km) of sinistral motion (bringing the northern portion outboard of the southern portion); high-flux magmatism prior to and during orthogonal mid-Cretaceous terrane accretion; low-flux magmatism during Late Cretaceous–Paleocene dextral transpressional motion; and high-flux Eocene magmatism during rapid exhumation in a regime of regional crustal extension.

Post–10 Ma uplift and exhumation of the northern Coast Mountains, British Columbia

Apatite (U-Th)/He ages reveal three distinct periods in the exhumation history of the northern Coast Mountains (∼54°N). A well-developed helium partial retention zone indicates little or no exhumation between ca. 30 and 10 Ma. Beginning at 10 Ma and extending to at least 4 Ma the range underwent steady but slow exhumation of ∼0.22 mm/yr, after which the exhumation rate increased by at least 70%. An 85-km-long He age traverse across the orogen at sea level shows that vertical offsets on post-10 Ma faults are minor. Furthermore, the sea-level He ages (14–2.8 Ma) inversely correlate with local mean elevation along the traverse. These data suggest that the Coast Mountains first appeared as a significant topographic feature only within the past few million years. This history is not consistent with suggestions that uplift of the range resulted from formation of the Queen Charlotte basin in early-middle Miocene time. We speculate that intense erosion by alpine and continental glaciation after 2.5 Ma triggered isostatic uplift and creation of the modern topography of the Coast Mountains.

Apatite (U-Th)/He signal of large-magnitude accelerated glacial erosion, southwest British Columbia

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.

Coast Plutonic Complex: A mid-Cretaceous contractional orogen

Early Late Cretaceous east-vergent thrusts deform much of the eastern margin of the Coast Plutonic Complex in western British Columbia. Similarities in timing and style suggest that the faults represent a system of backthrusts to a west- vergent thrust belt on the west side of the complex. This geometry and the rarity of mid-Cretaceous strike-slip faults indicate that the Coast Plutonic Complex was a strongly contractional orogen in mid-Cretaceous time.

Evolution of the eastern Waddington thrust belt and its relation to the mid-Cretaceous Coast Mountains arc, western British Columbia

The eastern Waddington thrust belt cuts the eastern margin of the Coast Mountains mid-Cretaceous magmatic arc. Thrust faults carried Triassic rocks of the Intermontane superterrane, Early Cretaceous volcanic and clastic rocks, and volcanic and plutonic rocks of the active arc to the northeast, outward from the core of the arc. Minimum shortening across the thrust belt is estimated as 50% or 40 km. Synkinematic through postkinematic metamorphism produced an inverted metamorphic gradient with the structurally higher magmatic arc as the likely heat source. Radiometric, thermochronologic, and structural data indicate that the thrust belt was active at 84 Ma and probably at 87 Ma and suggest that the peak of postkinematic metamorphism occurred about 82–84 Ma. The thrust belt was intruded by postkinematic plutons in latest Cretaceous and early Tertiary time (68 Ma and 58 Ma). The eastern Waddington thrust belt is coeval with or slightly younger than a system of west directed thrusts in the western and southern parts of the Coast Mountains arc. The prominence of these structures suggests that synmagmatic contraction played a major role in development of the arc. The distribution and character of syn-and postthrusting metamorphism indicate that this contraction, rather than localized loading by magma, produced the metamorphism.

Southern continuation of the Coast shear zone and Paleocene strain partitioning in British Columbia-Southeast Alaska

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.

Middle Jurassic terrane accretion along the western edge of the Intermontane superterrane, southwestern British Columbia

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.

Late Cretaceous evolution of the eastern Coast Mountains, Bella Coola, British Columbia

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.

Paleogeography of the Insular and Intermontane terranes reconsidered: Evidence from the southern Coast Mountains Batholith, British Columbia

New geologic and paleomagnetic data from Knight Inlet in the southwestern Coast Mountains Batholith, British Columbia, support signifi cant revision to the paleogeography of the Insular and Intermontane terranes. Recompilation of radiometric ages confi rms that after 100 Ma, a magmatic arc migrated northeastward across the Coast Mountains Batholith at ~2 km/m.y. Magmatic age patterns suggest that plutons older than 100 Ma intruded the Intermontane terrane, not the expected Insular terrane. The distribution of brittle faults along Knight Inlet defi nes a structurally intact central domain, ~45 km wide, fl anked to the SW and NE by faulted domains, with no evidence of the widespread Tertiary extension affecting the batholith farther north. Al-in-hornblende geobarometry yields emplacement depths of ~2.5‐4 kbar and does not reveal systematic postemplacement tilting. Plutons in the central structural domain yield a consistently oriented paleomagnetic remanence presumably acquired as the Late Cretaceous arc cooled from ca. 110 to 85 Ma. In the absence of recognizable tilting, this result indicates ~1700 km of northward translation since ca. 85 Ma, which is signifi cantly less than predicted for the Insular terrane in the “Baja British Columbia” model but similar to results from the Intermontane terrane. The pluton ages and the paleomagnetic results suggest that the Intermontane terrane, not the Insular terrane, underlies the southwestern fl ank of the Coast Mountains Batholith. This conclusion is compatible with a paleogeographic model in which the Vancouver Island fragment of Wrangellia was juxtaposed against the Intermontane terrane prior to ca. 120‐100 Ma and emplaced in southern British Columbia after ca. 75 Ma.

Deformation of continental crust along a transform boundary, Coast Mountains, British Columbia

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 <9 Ma. Local tilting in the core of the orogen is associated with glacially driven, post-4 Ma exhumation. Integration with previous results shows that post-50 Ma dextral and normal faulting affected a region ~250 km inboard of the transform and ~300 km along strike. Initially widespread, the zone of active extension narrowed and migrated toward the transform ~25 Ma after initiation of the transform, while dextral faulting continued throughout the region. Differential amounts of post-50 Ma extension created oroclines at the southern and northern boundaries of the deformed region. This region approximately corresponds to continental crust that was highly extended just prior to transform initiation. Variation in Neogene crustal tilts weakens interpretations relying on uniform tilting to explain anomalous paleomagnetic inclinations of mid-Cretaceous plutons. Similarities to the Gulf of California suggest that development of a transform in continental crust is aided by previous crustal extension and that initially widespread extension narrows and moves toward the transform as the margin develops.