Abstract

The remanent magnetization of Paleozoic carbonates in the Front Ranges and Inner Foothills of the southern Canadian Rockies is remarkably constant along a 500-km strike length, sampled at 124 sites through four transects. Primary Paleozoic remanent directions, which should have shallow inclinations, are never observed. Rather, the paleomagnetic signal is dominated by geographically persistent remagnetizations, characterized by steep inclinations. In addition to a soft present-field overprint, we observed two distinctive secondary magnetizations, named the A and B components, carried by fine-grained magnetite. Pervasive diagenesis induced the A component, a total chemical remanent remagnetization. Poles for the A component are better concentrated after bedding correction indicating a predeformational or early syn-deformational origin. With only one exception, the A component has normal polarity in the Front Ranges and reverse polarity in the Inner Foothills. Pole positions, polarity, and geologic and thermal constraints indicate that the A component was acquired diachronously in advance of the eastward-migrating Cordilleran tectonic wedge. Subsequently, an intermediate temperature, partial thermoremanent remagnetization, the B component, was superposed on large regions of the Front Ranges and Inner Foothills. B component directions are brought into optimal concentration by differential untilting of 0% to 50%, indicating that the component was acquired after the rocks were incorporated into the orogenic wedge but before the end of contractional deformation. The B component is strongest within ∼2 km of the frontal thrust of the Front Ranges. The relative magnitude of the B to A components and the maximum unblocking temperature of the B component decrease away from the frontal thrust over a distance of about 30 km both to the west and to the east. The B component thermal overprint was attained by heating to <250 °C in response to tectonic or possibly sedimentary loading. It was preserved by a rapid cooling accompanying a differential uplift and erosion of up to 8 km in the vicinity of the frontal thrust late in or postdating its local tilting history. The likely cause was uplift of the exposed structural panel by contraction of younger underlying thrust structures.

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