The western quarter of North America consists of accreted terranes--crustal blocks added over the past 200 million years--but the reason for this is unclear. The widely accepted explanation posits ...that the oceanic Farallon plate acted as a conveyor belt, sweeping terranes into the continental margin while subducting under it. Here we show that this hypothesis, which fails to explain many terrane complexities, is also inconsistent with new tomographic images of lower-mantle slabs, and with their locations relative to plate reconstructions. We offer a reinterpretation of North American palaeogeography and test it quantitatively: collision events are clearly recorded by slab geometry, and can be time calibrated and reconciled with plate reconstructions and surface geology. The seas west of Cretaceous North America must have resembled today's western Pacific, strung with island arcs. All proto-Pacific plates initially subducted into almost stationary, intra-oceanic trenches, and accumulated below as massive vertical slab walls. Above the slabs, long-lived volcanic archipelagos and subduction complexes grew. Crustal accretion occurred when North America overrode the archipelagos, causing major episodes of Cordilleran mountain building.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Plate reconstructions since the breakup of Pangaea are mostly based on the preserved spreading history of ocean basins, within absolute reference frames that are constrained by a combination of ...age‐progressive hotspot tracks and paleomagnetic data. The evolution of destructive plate margins is difficult to constrain from surface observations as much of the evidence has been subducted. Seismic tomography can directly constrain paleotrench locations by imaging subducted lithosphere in the mantle. This new evidence, combined with the geological surface record of subduction, suggests that several intraoceanic arcs existed between the Farallon Ocean and North America during late Mesozoic times—in contrast to existing quantitative models that typically show long‐lived subduction of the Farallon plate beneath the continental margin. We present a continuously closing plate model for the eastern Pacific basin from 170 Ma to present, constrained using “tomotectonic analysis”—the integration of surface and subsurface data. During the Middle to Late Jurassic, we show simultaneous eastward and westward subduction of oceanic plates under an archipelago composed of Cordilleran arc terranes. As North America drifts westward, it diachronously overrides the archipelago and its arcs, beginning in the latest Jurassic. During and post‐accretion, Cordilleran terranes are translated thousands of kilometers along the continental margin, as constrained by paleomagnetic evidence. Final accretions to North America occur during the Eocene, ending ~100 Myr of archipelago override. This model provides a detailed, quantitative tectonic history for the eastern Pacific domain, paving the way for tomotectonic analysis to be used in other paleo‐oceanic regions.
Plain Language Summary
Tectonic plate reconstructions back to the Jurassic period are mostly based on data from preserved ocean crust. However, many oceanic plates have been lost into the Earth's interior by subduction, making their reconstruction a challenge. We combine seismic images of the deep mantle with geological data to locate the vanished plates in the Earth's mantle and restore them to their previous positions at the surface. The trenches where the plates subducted can be paired with extinct arc volcanoes in western North America. Jointly, these lines of evidence suggest that the eastern Pacific basin was broken up into several smaller plates—in contrast to the long‐held view that one or two large plates subducted eastward beneath the west coast of North America for the last 170 Myr. Instead, we model simultaneous eastward and westward subduction of plates under a vast archipelago of volcanic arcs that sat stationary in the northeastern proto‐Pacific. North America was pulled westward into this archipelago and gradually collided with its microcontinents, which today form the North American Cordillera. Our model provides a more detailed and complete tectonic history for the eastern Pacific and highlights how our method can be used to reconstruct vanished oceans.
Key Points
We present a plate reconstruction model for western North America and the eastern Pacific for the past 170 Myr
The plate model uses “tomotectonic analysis”—a synthesis of mantle and geological evidence
Detailed terrane motions show the assembly of the North American Cordillera from an intraoceanic arc setting
We analyze mantle structure under South America in the DETOX‐P1 seismic tomography model, a global‐scale, multifrequency inversion of teleseismic P waves. DETOX‐P1 inverts the most extensive data set ...of broadband, waveform‐based traveltime measurements to date, complemented by analyst‐picked traveltimes from the ISC‐EHB catalog. The mantle under South America is sampled by ∼665,000 cross‐correlation traveltimes measured on 529 South American broadband stations and on 5,389 stations elsewhere. By their locations, depths, and geometries, we distinguish four high‐velocity provinces under South America, interpreted as subducted lithosphere (“slabs”). The deepest (∼1,800–1,200 km depth) and shallowest (<600 km) slab provinces are observed beneath the Andean Cordillera near the continent’s northwest coast. At intermediate depths (1,200–900 km, 900–600 km), two slab provinces are observed farther east, under Brazil, Bolivia and Venezuela, with links to the Caribbean. We interpret the slabs relative to South America’s paleo‐position over time, exploring the hypothesis that slabs sank essentially vertically after widening by viscous deformation in the mantle transition zone. The shallowest slab province carries the geometric imprint of the continental margin and represents ocean‐beneath‐continent subduction during Cenozoic times. The deepest, farthest west slab complex formed under intra‐oceanic trenches during late Jurassic and Cretaceous times, far west of South America’s paleo‐position adjoined to Africa. The two intermediate slab complexes record the Cretaceous transition from westward intra‐oceanic subduction to eastward subduction beneath South America. This geophysical inference matches geologic records of the transition from Jura‐Cretaceous, extensional “intra‐arc” basins to basin inversion and onset of the modern Andean arc ∼85 Ma.
Plain Language Summary
Eastward subduction of the Pacific basin lithosphere beneath South America has generated arc magmatism and produced the modern Andes. However, extrapolation of the modern east‐dipping subduction scenario to before the Late Cretaceous does not readily explain the contrasting history of the ancestral Andes. Subducted former seafloor continues to exist in the mantle and remains visible to seismic tomography because P waves travel faster in it than in ambient mantle. We analyze such seismically fast domains, that is, “slabs” of interpreted paleo‐seafloor, at depths of ∼300–1,800 km in our global tomography model DETOX‐P1. Combining these observations with quantitative plate reconstructions and geological observations, we attempt to reconstruct the subduction history under South America. The slab that dips eastward down beneath the present‐day Andes is relatively continuous to ∼900 km depth. Deeper down, slab geometries completely change. Voluminous slabs are unexpectedly imaged thousands of kilometers west of South America’s reconstructed paleo‐margin. We argue that in the simplest explanation all slabs sank essentially in place, and a trans‐American, tectonic reconfiguration occurred the time equivalent to 900 km slab depth (∼80–90 Ma). Prior to this time, the “Andean” trench sat offshore (and the margin extended) but the subduction direction must have been oceanward, rather than eastward.
Key Points
Global‐scale teleseismic P wave tomography model DETOX‐P1 is analyzed under South America
The shallowest and the deepest slabs are found under (north)west South America, while intermediate‐depth slabs are found farther east
If slabs sank roughly vertically, they initially formed under intra‐oceanic trenches, up until Late Cretaceous times
On Earth, the velocity at which subducting plates are consumed at their trenches (termed “subduction rate” herein) is typically 3 times higher than trench migration velocities. The subduction rate is ...also 5 times higher than estimated lower mantle slab sinking rates. Using simple kinematic analyses, we show that if this present‐day “kinematic state” operated into the past, the subducting lithosphere should have accumulated and folded beneath near‐stationary trenches. These predictions are consistent with seismic tomography, which images localized and widened lower‐mantle slab piles. They are, however, at odds with most dynamic‐subduction models, which predict rapid trench retreat and inclined slabs in the mantle transition zone. We test the hypothesis that a weak asthenospheric layer (WAL), between the lithosphere‐asthenosphere boundary and 220 km depth, compatible with geophysical constraints, can remedy the discrepancies between numerical models and observations. The WAL lubricates the base of the lithosphere, increases the subduction rate while reducing trench retreat. As a consequence, simulations featuring a WAL predict slab accumulation at the mantle transition zone, and thicker, folded slabs in the lower mantle. A WAL viscosity only 2–5 times lower than that of the adjacent mantle is sufficient to shift subduction regimes toward a mode of vertical slab sinking and folding beneath near‐stationary trenches, across a wide range of model parameters, producing surface and slab velocities close to those observed at the present‐day. These findings provide support for the existence of a weak asthenosphere beneath Earth's lithosphere, complementing independent evidence from various geophysical data.
Plain Language Summary
At convergent margins (subduction zones, marked by deep trenches), oceanic (subducting) plates plunge into Earth's mantle. Analysis of the present‐day surface velocities suggest that subducting plates are consumed at trenches at rates of 5 cm/yr, on average. Moreover, it is observed that the consumption rate is higher than the trench migration velocity, which is often less than 1 cm/yr. At depth below 660 km, marking the transition from the upper to the lower mantle, the subducted piece of the plate (the slab) encounters increased resistance to its sinking, with slab sinking velocities at these depths being less than about 1.5 cm/yr. Take together, such rapid plate consumption at quasi‐fixed trenches, along with slab deceleration in the lower mantle, causes a “traffic jam” leading to sub‐vertical accumulation of the slab and folding. This behavior is confirmed by seismic imaging techniques of Earth's interior which reveals vertically‐sinking piles of oceanic slabs at and beneath a 660‐km depth. However, computational and laboratory models of subduction zones often fail to reproduce these first‐order observations. Here, we demonstrate that the addition of a lubricating mantle layer at the base of the oceanic plates reduces the mismatch between the aforementioned observations and predictions from 2‐D computation models.
Key Points
Tectonic plate kinematics and seismic tomography suggest slab accumulation in the mantle transition zone, beneath near‐stationary trenches
By contrast, subduction dynamics models tend to produce inclined, laterally extended slabs associated with slab rollback and trench retreat
Adding a sub‐lithospheric weak layer accelerates subduction, limits trench migration, and promotes sub‐vertical slab piles, as observed
Onset and termination of Eocene felsic volcanism in the Chilcotin Plateau of central British Columbia is constrained between 54.6 and 46.6 Ma by 33 new U-Pb and 40Ar/39Ar isotopic age determinations. ...Dates were obtained from representative felsic coherent and fragmental volcanic rocks that comprise the Ootsa Lake Group. The resulting chronostratigraphy shows that magma compositions evolved from felsic to intermediate, with no spatial migration of the volcanic activity. Rhyolitic compositions are oldest; and are overlain by dacitic rocks with varied phenocrysts assemblages. In many parts of the Chilcotin Plateau, the Eocene stratigraphy is capped by distinctive vitreous black dacite lavas, which are contemporaneous with andesitic lavas of the Endako Group in the Nechako Plateau to the north. Crystallization ages from Ootsa Lake Group rocks of the Chilcotin Plateau overlap age determinations from correlative rocks of the Nechako Plateau and southern BC. Collectively, this geochronological dataset supports previous suggestions of a voluminous Early Eocene-aged (∼55-46 Ma) period of volcanism in the Intermontane Belt. The abrupt initiation of volcanism, as well as the wide extent, thickness, and compositions that characterize Eocene volcanic rocks may be explained by cessation of subduction and formation of a slab gap beneath British Columbia in the Early Eocene.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The northern Cache Creek terrane in the Canadian Cordillera includes a subduction complex that records the existence of a late Paleozoic - Mesozoic ocean basin and provides an opportunity to assess ...accretionary processes that involve the transfer of material from a subducting plate to an upper plate. Lithogeochemical data from basaltic rocks indicate that the northern Cache Creek terrane is dominated by two different petrogenetic components: (1) a dominant suite of subalkaline intrusive and extrusive rocks mostly of arc affinity and (2) a volumetrically less significant suite of alkaline volcanic rocks of within-plate affinity. The subalkaline intrusive and extrusive rocks constitute a section of oceanic lithosphere that is interpreted to have occupied a fore-arc position during the Late Triassic and Early Jurassic before it was accreted during collisional orogenesis in the Middle Jurassic. Alkaline volcanic rocks in the northern Cache Creek terrane are stratigraphically associated with carbonate strata that contain Tethyan fauna that are exotic with respect to the rest of North America; together, they are interpreted as remnants of oceanic seamounts and (or) plateaux. The volcanic rocks are a minor component of the carbonate stratigraphy, and it appears that the majority of the volcanic basement was either subducted completely at the convergent margin or underplated at greater depth in the subduction zone. In summary, accretion in the northern Canadian Cordillera occurred principally by the accretion of island arcs and emplacement of fore-arc ophiolites during collisional orogenesis. The transfer of oceanic sediments and the upper portions of oceanic seamounts from the subducting plate to an accretionary margin accounts for only small volumes of growth of the upper plate.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A Paleozoic volcanic assemblage exposed in northern British Columbia, near the Turnagain River, previously considered to be part of an accreted terrane, was reported to be in depositional contact ...with a part of the Cordilleran miogeocline. This paper presents an integrated field, U-Pb geochronology, Sm-Nd isotopic, and geochemical study across the basal contact of the volcanic assemblage. Strongly evolved ?Nd(T) values, between -13 and -21 from samples of lower Paleozoic sedimentary rocks exposed below the volcanic rocks, and correlated with Atan - Kechika - Road River - Earn strata of the miogeocline farther east, support a North American miogeoclinal affinity, consistent with previously established regional stratigraphic and structural relationships. Nd isotopic data from the volcanic assemblage contrast significantly with data from the sedimentary rocks and record a mantle source (εNd(T) values between +4.0 and +7.0), consistent with a magmatic arc or back arc; negative Nb anomalies are similarly compatible with either arc-or back-arc-related magmatism. A concordant 339.7±0.6 Ma U-Pb zircon date was obtained from the volcanic assemblage. The mixed gradational contact between the miogeoclinal and volcanic rocks is marked by interlayering of finely laminated grey and green phyllites on the scale of centimeters, with no evidence of a tectonic contact. Bedding at the contact is folded into tight outcrop-scale folds that are intruded by an Early Jurassic (187.5±2.9 Ma) granodiorite. On the basis of all available evidence, the contact is interpreted as a facies transition. The Mississippian volcanic assemblage may link the miogeocline with the early development of an Angayucham-Slide Mountain basin.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We examine controls on mantle oxygen fugacity (
fO
2) during the partial melting process that forms mantle lithosphere at spreading centers. We compare the paleo
fO
2 at the time of melting inferred ...by V/Sc systematics of ophiolite peridotites, with the thermobarometric
fO
2 recorded by olivine–orthopyroxene–spinel assemblages during simple cooling in relatively young oceanic lithosphere. Modelling of the V/Sc in the ophiolite peridotites from Alaska, Yukon and British Columbia is permissive of only a narrow range in log
fO
2 during melting between NNO and NNO
−
1 (where NNO is the nickel–nickel oxide buffer), depending on the choice of partition coefficients for Sc. This result is within uncertainty of the thermobarometric
fO
2 recorded by most samples (within 1 log unit of NNO
−
1). The same cannot be said for more complex peridotite residues from continental mantle, where V/Sc systematics show a narrow paleo
fO
2 during formation but wide range of thermobarometric
fO
2 after equilibration in the lithosphere. In continental mantle with a complex history, thermobarometric
fO
2 is an ambiguous measure of that attendant during partial melting. Graphite-saturated melting in a system closed to oxygen controls melt Fe
3+/Fe
2+and CO
2 content, and creates a shift in
fO
2 of about 2 log units
1 in a peridotite residue. In contrast, for the ophiolite mantle samples in this study, both paleo- and thermobarometric
fO
2 are near values predicted by carbon–fluid equilibria, yet show no relationship with depletion, suggesting the melt-residue system in the mantle may be open to oxygen during the partial melting process.
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