ALLEGHANIAN-DRIVEN FLUID MIGRATION ACROSS THE APPALACHIAN BASIN INTO THE MICHIGAN BASIN

A compilation of thermal maturation indicies across the Michigan Basin indicates the heat flux in the southeast portion of the basin was anomalously high at the time of maximum burial. A high thermal gradient in this area can be only partially attributed to systematic variations in lithologies (e.g., the shale-rich basin margins have lower thermal conductivities than the evaporite-rich interior). However, lithologic variations are not sufficient to account for the magnitude of the paleothermal gradient, nor the localization of the elevated gradient along only the southeast portion of the basin. In addition, published heat flux data from the basement indicate lower fluxes in this region than in surrounding areas.

Without an obvious source of conductive heat, advective heat transport to this area represents a plausible alternative to explain the elevated gradient. This proposed mechanism is consistent with numerical modeling results and is strongly supported by circumstantial evidence. In this scenario, the topographic head provided by the late Paleozoic Alleghanian orogeny drove deeply circulating brines northwestward out of the Appalachian Basin along basal Cambrian sandstones. To match the observed paleothermal gradient, the advected heat would have had to increase the heat flux approximately 30% above ambient conditions in southeast Michigan. Modeling indicates that a topographic head of approximately 10,000 ft along the Appalachian Mountains would have provided sufficient drive for fluids to impact the thermal gradient in southeast Michigan. According to previous estimates, such elevations were likely during the Alleghanian orogeny as the Appalachians may have approached Himalayan heights. More importantly, radiometric dating and reset paleomagnetic pole positions of diagenetic minerals within the Appalachian Basin strongly supports the existence of advective heat flux during Pennsylvanian-Permian time. Also, fluid inclusion evidence in Silurian and younger strata along the Findley Arch documents the presence of warm (+120ºC) brines on the margin of the Michigan Basin. Although direct documentation is unavailable, modeling and indirect evidence point to trans-basinal advective heat flux to account for the unusual paleothermal gradient in the southeast portion of the Michigan Basin.