2 Structural Geology and Tectonics (Posters)

Thursday, 13 March 2003
8:00 AM - 12:00 PM

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

Paper number: 2-8
Program: Technical Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

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, N33oE, vertical to steeply-southeast-dipping metamorphic foliation. The shear zones generally strike N67oE 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.