A MID-CRETACEOUS PLATEAU OF ALTIPLANO PROPORTIONS IN THE NORTH AMERICAN CORIDLLERA - WAS IT DEFLATED BY DEEP CRUSTAL FLOW?
In the mid- to Late Cretaceous there was a major orogen-wide compressional event within the southern Canadian Cordillera, coinciding with final accretion of Insular terranes. Plutonism, deformation and high-grade metamorphism that accompanied this orogenic event are most obviously manifest in the Coast and southern Omineca belts where crustal thicknesses were 55-65 km, similar to that of the Altiplano plateau. Was there a continuous ~4-5 km high plateau across the width (~500 km) of the west-central North American Cordillera, or was there an intervening low-lying area of subdued topography much like there is today? To answer this question we need to first restore the displacement along Paleogene strike-slip faults. This places the Bowser basin in the region between the Coast and southern Omenica belts in the mid- to Late Cretaceous. At this time, the geology of the Bowser basin indicates a thickness of ~40-45 km, implying a crustal architecture akin to lithospheric-scale boudinage.
Alternatively, if there was an Altiplano-sized plateau across this part of the North American Cordillera, how do we explain the apparently thinner crust underlying the Bowser basin? I propose that extension in the upper crust accompanied by general non-coaxial flow within the deepest levels of the plateau could have produced accommodated crustal thinning within the central region. In this scenario, the general non-coaxial flow, with a significant pure-shear component, moved material from beneath the central region of the plateau, which accentuated thickening of the flanking regions, resulting in the pair of high-standing orogenic welts that we see today.