Download Cordilleran foreland vs hinterland deformation: Thermal controls of

Cordilleran foreland vs hinterland deformation:
Thermal controls of deformation style
T.J. Lewis*
(Sidney Geophysical Consultants Ltd., Sidney, B.C.,
1107 Maple Road, Sidney, B.C. V8L 5P5, [email protected]
Also Emeritus, Pacific Geoscience Centre, G.S.C.)
R.D. Hyndman
(Pacific Geoscience Centre, Geological Survey of Canada)
P. Flueck
(School of Earth and Ocean Sciences, University of Victoria)
Introduction
The Canadian Rocky Mountain fold and thrust belt exhibits the characteristic orogen division into foreland
and hinterland deformation styles. In the foreland the deep crust and upper mantle have remained largely
undeformed during late Mesozoic to early Tertiary tectonics. Shortening occurred primarily in overlying
sedimentary thrust sheets, mediated by high pore fluid pressures (thinned skin tectonics). A large foreland
basin has formed. In contrast, Cordillera hinterland shortening deformation and subsequent extension
involved the crystalline crust (thick skinned tectonics). The contrast is explained by the pronounced
difference in lithosphere temperature and thus strength between the two regions. In the southern
Canadian Cordillera the thermal transition occurs within the mountain belt near the Rocky Mountain
Trench about 100 km west of the mountain front. In the Yukon-NWT northern Cordillera, the transition is
not yet as well defined, but appears to occur near the mountain front. The thin foreland basin in the north
may imply that the thrust thickening of the sedimentary section has not reached and loaded the western
edge of the cold Craton lithosphere.
Thermal constraints
The primary data that constrain deep crust and upper mantle temperatures are geothermal heat flow and
radioactive heat generation measurements. The Craton-Cordillera thermal contrast is clearly evident in
the large difference in heat flow, 40-60 mW/m2 in the stable Craton, and 80-120 mW/m2 in most of the
Cordillera. The Cordilleran heat flow is remarkably high everywhere except for near the southwest coast
where heat flow is reduced by the subducting Juan de Fuca plate. Several other temperature indicators
provide additional constraints on the deep temperatures and help to define the location of the CratonCordillera deep thermal boundary, (1) temperature-dependent uppermost mantle Pn velocities (Pn~8.2 for
the cold Craton, Pn~7.8 for the hot Cordillera), (2) crustal thickness and isostasy balance (discussed
below), (3) mantle xenoliths that indicate much cooler upper mantle and deep crust temperatures for the
Craton. The estimated temperatures at the Moho are 400-500oC for the Craton. The temperatures at the
Moho for most of the Cordillera are remarkably high, 800-1000oC. In the southern Cordillera, the high
heat flow appears to result from upward and westward advection of heat in the subduction backarc,
driven by the downgoing plate. Surprisingly, the heat flow is equally high in the northern Cordillera, where
subduction ceased in the Eocene with the formation of the Queen Charlotte-Fairweather transform fault.
Advection of heat to the former backarc seems to persist for a long time.
Thermal controls of foreland vs hinterland deformation
Craton-Cordillera isostatic balance
Along the length of the western Canada mountain belt, isostatic balance is maintained across the CratonCordillera thermal boundary by thermal expansion and density reduction of the lithosphere beneath the
Cordillera. However, there is an important contrast between the north and south. In the southern
Canadian Cordillera, there is no significant change in elevation across thermal boundary located within the
mountain belt near the Rocky Mountain Trench, but there is a pronounced change in crustal thickness.
The Craton crust to the east is 40-50 km thick whereas the Cordillera crust to the west is 32-34 km thick.
It is concluded that thermal density reduction beneath the Cordillera balances the thinner crust. In
northern British Columbia-Yukon, at the thermal boundary near the mountain front, there is almost no
change in crustal thickness but there is a change elevation from near sealevel in the east to the much
higher mountains to the west. Across this boundary the higher elevation of the Cordilleran mountain belt
is maintained in isostatic balance with the lower elevation Craton through the temperature effect on
lithosphere density.
Lithosphere thickness, strength and style of deformation
The Canadian Rocky Mountain fold and thrust belt exhibits the characteristic orogen division into foreland
and hinterland deformation. In the foreland the deep crust and upper mantle have remained largely
undeformed during late Mesozoic to early Tertiary tectonics. Shortening occurred primarily in overlying
sedimentary thrust sheets, mediated by high pore fluid pressures, i.e., thinned skin tectonics. In contrast,
Cordillera hinterland shortening deformation (and subsequent extension mainly in the south) involved the
crystalline crust, i.e., thick skinned tectonics. The contrast is explained by the pronounced difference in
lithosphere temperature and thus strength between the two regions.
Using estimated temperatures in the crust and upper mantle, the lithosphere thicknesses and strengths
may be predicted based upon laboratory determined rheologies for crustal and mantle rocks as a function
of temperature. The consequent thicknesses for the lithosphere (i.e., depth of significant strength), are
approximately defined by ~450oC for crustal and ~750oC for mantle compositions. The lithosphere
thickness are only the upper ~15 km of the crust for the Cordillera west of the thermal boundary, but over
100 km for the Craton. The lower crust and upper mantle beneath the Cordillera have negligible strength.
The total Craton lithosphere is at least 10 times stronger than that of the Cordillera. As a consequence,
tectonic lithosphere deformation is limited to the high temperature hinterland. In the southern Canadian
Cordillera, as well as shortening, the upper Cordillera crust (i.e., whole Cordillera “lithosphere”) has been
thrust over the Craton. Detachment near the base of the sediment section in the Foreland Belt appears to
continue westward into the Cordillera and merge with the Monashee decollément at a crustal depth of 1520 km, linking shortening across the orogen.
Foreland basins, south and north
In the southern Canadian Cordillera the deep thermal transition occurs near the Rocky Mountain
Trench, about 100 km west of the mountain front. In this area the upper Cordillera crust (i.e., whole
Cordillera “lithosphere”) has been both shortened and thrust ~100 km over the strong Craton crust that is
estimated to extend westward about to the Rocky Mountain Trench (thinned and heated craton mantle
lithosphere may extend further west). The thick thrust sheets pushed in front of the advancing Cordilleran
upper crust have loaded the strong Craton lithosphere. Because the Craton lithosphere is very cold and
Thermal controls of foreland vs hinterland deformation
thick, it has a long flexural wavelength and a very wide foreland basin resulted. In the north, a much
thinner foreland basin has formed which suggests that the thrust thickening of the sedimentary section has
not yet reached and significantly loaded the western edge of the cold Craton lithosphere. This may be
because there is less shortening in the north compared to the south. The deep lithosphere thermal
boundary is interpreted to be near the mountain front. Strong seismicity near the northern mountain front
indicates that thrusting is continuing, perhaps because of the collision of Yakutat terrane along the west
coast.
Biographical Notes:
Trevor Lewis is a private contractor mainly involved in global land and marine geothermal measurements.
He also is emeritus scientist at the Pacific Geoscience Centre of the Geological Survey of Canada. In his
career with the Geological Survey he worked on geothermal studies in many areas of Canada. He is
responsible for most of the heat flow and radioactive heat generation data in the Canadian Cordillera that
provide the primary constraints on deep temperatures.
Roy Hyndman is a Research Scientist at the Pacific Geoscience Centre of the Geological Survey of
Canada, and an Adjunct Professor at the School of Earth and Ocean Sciences, University of Victoria.
He is a geophysicist involved in tectonics, structure and earthquakes, especially in western Canada and
offshore.
Paul Flueck is a PhD student at the School of Earth and Ocean Sciences, University of Victoria,
carrying out thesis research on tectonics in western Canada.