CAN THE GRENVILLE FRONT BE TRACED SOUTHWARD INTO ALABAMA?

The Grenville front has been traced using potential fired data from its exposure along the N side of Georgian Bay in Ontario southward in the subsurface into western OH where it is imaged in a COCORP seismic reflection line. In exposure and in seismic images in OH, the Grenville front is a gently E-dipping thrust that likely formed during the Rigolet phase of the Grenville orogeny, transporting high-grade rocks of the Grenville orogen westward. The boundary has also been traced across KY using drillhole, potential field, and some seismic reflection data, so the boundary should be traceable southward into Tennessee. Limited drillhole data in Tennessee provide some indication that the front is present in TN. A basement drillhole in Old Hickory, TN, yielded a discordant upper intercept SIMS zircon age of 1,381 + 27 Ma of a coarse-grained granitoid typical of the Mid-Continent granite-rhyolite province. A basement drillhole N of Crab Orchard, TN, bottomed in post-Grenville(?) undeformed mafic rocks and have not been dated. A basement drillhole near Carthage, TN, bottomed in altered, undeformed partially altered basalt, rhyolite, and amygdaloidal basalt. The core from this hole was inadvertently mostly destroyed, but some fragmentary basement core was recovered. Both the Carthage and Crab Orchard basement holes were drilled into extensive magnetic-gravity highs.

The Grenville front can likely be traced across TN with some certainty using the limited drill data in concert with potential field data. A weak magnetic trend in eastern AL could mark the Grenville front, but that is uncertain, except that this trend roughly lines up with the E-W trace of the Grenville front across Central TX. The Grenville front could also bifurcate into at least two other possible traces in the AL aeromagnetic and 50-km high-pass gravity data, so S of TN the location of the Grenville front highly speculative. It likely crosses Tennessee and can be speculated to pass through AL, MS, and LA into TX where it has been traced with greater certainty N of the Llano uplift.