Abstract of paper presented at CSPG – SEPM Joint Convention, June 1997, Calgary, Alberta

Paleomagnetic constraints on fluid migration, tectonic, and thermal histories in triangle zones of southern Alberta and the Arkoma Basin

D. R. Van Alstine and J. E. Butterworth, Applied Paleomagnetics, Inc.
S. R. May and M. C. Covey, Exxon Production Research Co.

Our paleomagnetic studies in the Foothills/Front Ranges of Alberta and in the Ouachita mountain front demonstrate the potential of using paleomagnetism to detect migration of fluid, chemical, thermal, and strain fronts through the “triangle zone” at the leading edge of fold-thrust belts. In the Foothills/Front Ranges, paleomagnetism can independently monitor the fluid-migration history via chemical remanent magnetization (CRM) and the burial/uplift history via viscous partial thermoremanent magnetization (VPTRM) relative to a distinctive change in geomagnetic “polarity bias” that occurs at ~63 Ma. By polarity-bias age-dating and by applying the paleomagnetic “fold test,” we can determine when remagnetizing “orogenic fluids” migrated through particular Foothills/Front Ranges structures, such as the fault-propagation fold at Mt. Kidd and the anticline at Moose Mountain.

In Alberta, the McConnell thrust, which marks the Front Ranges/Foothills boundary, also marks a profound paleomagnetic discontinuity: the hanging wall contains the normal-polarity, >63 Ma, pre-folding CRM predominant throughout the Front Ranges, and the footwall contains the reversed-polarity, <63 Ma, synfolding/post-folding CRM predominant throughout the Foothills. In the Arkoma basin, the Choctaw thrust marks an important paleomagnetic boundary: in surface outcrops behind the Choctaw thrust, an early-orogenic, “pre-folding” 305 Ma CRM residing in pyrrhotite±magnetite indicates sulfur-rich/oxygen-poor redox conditions within an older, “relict” triangle zone, whereas in cores from subsurface wells ahead of the Choctaw thrust, a late-orogenic, “post-folding” CRM residing in magnetite±maghemite indicates sulfur-poor/oxygen-rich chemical conditions within a younger “incipient” triangle zone.

Chemical remagnetization by “orogenic fluids” can be intimately related to fracturing and rock-mechanical properties. In the Arkoma basin, the most strongly remagnetized rocks contain a late-stage carbonate or silica cement that greatly influences natural and present-day hydraulic fractures.

Investigation of VPTRM yields important insights into the burial/uplift history, independent of the fluid-migration history recorded by CRM. The hanging walls of the Lewis thrust at Mt. Kidd and the McConnell thrust at Compression Ridge record reversed-polarity, <63 Ma, west-tilted VPTRMs, probably reflecting cooling upon uplift on the west flank of an “antiformal imbricate stack” within the triangle zone. In the eastern Arkoma basin, the 305 Ma, reversed-polarity CRM acquired during the Ouachita orogeny has been partially overprinted by an 85 Ma, normal-polarity VPTRM, consistent with a regional Late Cretaceous uplift/cooling event corroborated by apatite fission-track analysis.