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TitleLower-crustal flow and detachment in the North American Cordillera: a consequence of Cordillera-wide high temperatures
AuthorHyndman, R DORCID logo
SourceGeophysical Journal International vol. 209, no. 3, 2017 p. 1779-1799,
Alt SeriesNatural Resources Canada, Contribution Series 20190046
PublisherOxford University Press (OUP)
Mediapaper; on-line; digital
File formatpdf (Adobe® Reader®); html
ProvinceYukon; Northwest Territories; British Columbia; Alberta
NTS82; 83; 92; 93; 94; 95; 102; 103; 104; 105; 106; 107; 114; 115; 116; 117
AreaCanada; United States of America; Mexico
Lat/Long WENS-160.0000 -90.0000 72.0000 14.0000
Subjectstectonics; igneous and metamorphic petrology; geophysics; Science and Technology; Nature and Environment; crustal evolution; tectonic environments; subduction zones; thermal history; thermal analyses; heat flow; temperature; crustal thickness; crustal movements; deformation; creep; shearing; faulting; geophysical interpretations; seismic interpretations; pressure-temperature conditions; viscosity; lithosphere; Mohorovicic discontinuity; mantle; rheology; strain analysis; modelling; North American Cordillera; Basin and Range; Cascadia Subduction Zone; Columbia Plateau
Illustrationsschematic cross-sections; seismic reflection profiles; geoscientific sketch maps; profiles; plots; models
ProgramGEM2: Geo-mapping for Energy and Minerals Western Cordillera, Devonian and Permian structural architecture
Released2017 04 26
AbstractIn this paper, I make the case for widespread lower-crustal detachment and flow in the North American Cordillera. An indicator that geologically recent flow has occurred comes from seismic structure data showing the crust in most of the Cordillera from Mexico to Alaska is uniformly thin, 33 ± 3 km, with a remarkably flat Moho. The flat Moho is in spite of extensive normal faulting and shortening that might be expected to deform the Moho. It has been concluded previously that the high topographic elevations are due to thermal expansion from Cordillera-wide high temperatures compared to stable areas, not due to a crustal root. I argue that the constant crustal thickness and flat Moho also are a consequence of temperatures sufficiently hot for flow in the lower crust. Lower-crust detachment and flow has previously been inferred for Tibet and the high Andes where the crust is thick such that unusually high temperatures are expected. More surprising is the similar conclusion for the Basin and Range of western USA where the crust is thin, but high temperatures have been inferred to result from current extension. There are now adequate data to conclude the Basin and Range is not unique in crustal thickness or in temperature. The crust in most of the Cordillera is similarly hot in common with many other backarcs. Five thermal constraints are discussed that indicate that for most of the Cordillera, the temperature at the Moho is 800-850 ºC compared to 400-450 ºC in stable areas. At these temperatures, the effective viscosity is low enough for flow near the base of the crust. The backarc Moho may be viewed as a boundary between almost 'liquid' lower crust over a higher viscosity, but still weak upper mantle. The temperatures are sufficiently high for the Moho to relax to a nearly horizontal gravitational equipotential over a few tens of millions of years. The inference of a weak lower crust also suggests that topography over horizontal scales of over 100 km must be short lived over a similar timescale, after the generating forces relax. A weak lower crust in the Cordillera is also shown by the effective elastic thickness, Te, which indicates significant strength only in the upper crust. Other indicators of lower-crust flow or detachment are seismic reflectors in the lower crust that are interpreted to result from horizontal shearing, and outcrop sections exhumed from the deep crust that exhibit horizontally sheared fabric.
Summary(Plain Language Summary, not published)
This article documents the evidence that there is large-scale both lower crustal detachment from the mantle, and lower crustal flow that flattens the Moho over much of the Cordillera. The implication is that most geological structures mapped at the surface may be substantially displaced from their mantle origin and control. This displacement may include many deep source mineral deposits. This conclusion also provides a mechanism for the current and past tectonic activity producing thrusting at the northern Cordillera mountain front of Mackenzie Mountains (and current large earthquakes)and the Beaufort-Mackenzie thrust belt, driven by processes at the Pacific continental margin.

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