TECONIC IMPLICATIONS OF A 369 MA U-PB ZIRCON DATE ON GRANITIC DIKES CUTTING RIGHT-SLIP DUCTILE SHEARS ASSOCIATED WITH THE ALEXANDER CITY FAULT ZONE, ALABAMA

We report U-Pb dates on zircons from granitic dikes that cut ductile shears in rocks of the Alabama Piedmont, documenting Early Ordovician – Late Devonian timing for right-slip shearing in the southernmost Appalachians. The shears occur within the Elkahatchee Quartz Diorite, a previously dated, 490 Ma, pre-metamorphic batholith that has intruded metasedimentary units of the eastern Blue Ridge. The Elkahatchee has been metamorphosed to sillimanite-zone conditions and contains a strongly developed, N33^oE, vertical to steeply-southeast-dipping metamorphic foliation. The shear zones generally strike N67^oE and dip steeply toward the southeast. They are difficult to recognize in the homogeneous quartz diorite but are spectacularly displayed where they cut xenoliths and pegmatitic veins. A right-lateral, strike-slip sense of rotation along these noncoaxial simple-shear zones is recorded by trains of sigma clasts, normal-slip crenulations, and locally well-developed S-C composite planar fabrics. Quartz and feldspar are crystal-plastically deformed indicating temperatures of deformation in excess of ~450 ^oC. We interpret the shear zones to be associated with the Alexander City fault zone that lies less than 1.5 km to the southeast of the outcroppings we investigated.

Narrow (<0.5 m thick), steeply-dipping, trondhjemitic dikes commonly cut across the shear zones at high angles. The dikes are lighter gray colored and finer grained than the quartz diorite. We analyzed U-Pb isotopes of zircons separated from a sample of one of the trondhjemite dikes (Elk-21) where it clearly cuts across several of the right-slip shears in the same outcropping. Sixteen zircon fractions were analyzed, of which eleven appear to be xenocrysts and five fall on concordia at 369.4 +1.6 and -1.4 Ma. We interpret the 369 Ma date to record the time of igneous crystallization, thus placing a minimum age on the time of right-slip shearing. We discuss the significance our findings have for pre-Alleghanian right-slip ductile faulting in the southern parts of the orogen, which appears remarkably similar kinematically and geometrically to the well-documented, Alleghanian right-slip system.