Orogenic remagnetizations in the front ranges and Inner Foothills of the southern Canadian Cordillera: Chemical harbinger and thermal handmaiden of Cordilleran deformation

The remanent magnetization of Paleozoic carbonates in the Front Ranges and Inner Foothills of the southern Canadian Rockies is remarkably constant alo ng a 500-km strike length, sampled at 124 sites through four transects, Pri mary Paleozoic remanent directions, which should have shallow inclinations, are never observed, Rather the paleomagnetic signal is dominated by geogra phically 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-gra ined magnetite. Pervasive diagenesis induced the A component, a total chemi cal remanent remagnetization. Poles for the A component are better concentr ated after bedding correction indicating a predeformational or early syn-de formational origin. With only one exception, the A component has normal pol arity in the Front Ranges and reverse polarity in the inner Foothills, Pole positions, polarity, and geologic and thermal constraints indicate that th e A component was acquired diachronously in advance of the eastward-migrati ng Cordilleran tectonic wedge, Subsequently; an intermediate temperature, p artial thermoremanent remagnetization, the B component, was superposed on l arge regions of the Front Ranges and Inner Foothills. B component direction s are brought into optimal concentration by differential untilting of 0% to 50%, indicating that the component was acquired after the rocks were incor porated into the orogenic wedge but before the end of contractional deforma tion, The B component is strongest within similar to 2 km of the frontal th rust of the Front Ranges. The relative magnitude of the B to A components a nd the maximum unblocking temperature of the B component decrease away from the frontal thrust over a distance of about 30 km both to the,vest and to the east. The B component thermal overprint was attained by heating to <250 degrees 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 struc tural panel by contraction of younger underlying thrust structures.