ARC, COLLISIONAL AND SLAB-FAILURE MAGMATISM WITHIN THE CORDILLERAN OROGEN OF NORTH AMERICA

The Cordilleran orogen of North America extends from southern Mexico to Alaska and is generally interpreted to represent an asymmetrical, bivergent, continental margin orogen formed by eastwardly-directed subduction of Pacific oceanic lithosphere beneath North America in which back-arc basin and continental margin rocks were thickened – in part by retrograde thrusting starting in the Jurassic – to spread eastward where they formed a critical-taper fold-thrust wedge with an associated foreland basin. However, this model glosses over much contrary data and so is unlikely to work.

Some of these problems and/or inconsistencies are: (1) the overall paucity of rift volcanics on the Cordilleran margin; (2) robust paleomagnetic data which indicate that much of the Cordillera west of the cratonic terrace did not ultimately dock with the craton until about 60-50 Ma; (3) Mesozoic plutonic rocks of the US Great Basin with isotopic compositions inconsistent with derivation from North American crust; (4) 1.2-1.0 Ga metamorphism and deformation of the Belt-Purcell metasedimentary rocks that are unknown in northwestern North America and suggest long-distance transport; (5) persistent and atypical Paleozoic alkaline magmatism in postulated continental slope-rise rocks from Alaska to Mexico and Belt-Purcell cover in Idaho-Montana; (6) Triassic conglomerate, containing clasts of eclogite and blueschist unconformably overlies rocks of the Cassiar platform and Selwyn basin, indicating thrust emplacement and terrane amalgamation in the Triassic, not the Jurassic; (7) continuous foredeep sedimentation on the North American platform didn’t start until about 124 Ma, indicating that the Panthalassic basin hadn’t yet closed; and (8) it fails to adequately account for the voluminous 120-80 Ma Cordilleran-type batholiths, the Laramide thick-skinned belt, and regional Cenozoic extensional tectonism and metamorphic core complexes.

Many of the difficult-to-explain features in the conjectural back-arc model are readily explained and integrated into an actualistic model if the orogeny was caused by failed westerly-directed subduction of the segmented western margin of North America beneath an arc-bearing superterrane, which I call Rubia. Prior to collision, magmatism within the upper plate was highly varied and reflected the complexities of terrane amalgamation. By the Jurassic, the newly consolidated Rubian superterrane became the site of continental arc magmatism, with most eruptions taking place in low-lying areas at or near sea level. This is typical of nearly all modern continental arcs, such as Kamchatka, the Alaskan Peninsula, Central America, Japan, the Cascades, New Zealand, Central America, and Italy, all of which are dominantly low standing, even though individual volcanoes might be very tall. At about 124 Ma – based on widespread deposition of intraplatformal gravels directly beneath the foredeep – the North American platform entered the trench beneath the superterrane, and the sediment-rich distal part of the margin started to dewater, which generated voluminous basalt that rose into the crust to produce the intense magmatic flare-up, crustal reorganization, and uplift typical of Cordilleran-type batholiths. This is in keeping with the observation that even very long-lived subduction of oceanic crust and its veneer of abyssal sediment cannot produce the intense magmatic flux necessary to create Cordilleran batholiths.

By 80 Ma the difficulty of subducting an old craton had slowed convergence such that convection in the asthenospheric wedge stalled and magmatism ceased. Within 5 my the North American plate failed, which not only led to the subduction of the transitional outer crust with its slope-rise sediments and rift-facies rocks; but also to (1) shutdown of the thin-skinned thrusting in the fold-thrust belt; (2) uplift, exhumation and gravitational collapse in the hinterland belt and in the Coast plutonic complex; (3) linear belts of slab break-off magmatism and associated porphyry copper deposits; and (4) thick-skinned basement-involved folding and thrusting within the Great Basin segment. The slab failure magmatism apparently died by 60 Ma, but by 53 Ma new easterly-directed subduction led to magmatism in the Canadian and Sonoran segments of the orogen. Subsequent magmatism reflected the interplay between eastward subduction and collapse of the orogenic highlands during the Miocene to produce the Basin and Range extensional province.