CHARACTERIZATION AND AGE OF PSEUDOTACHYLITES AT THE MARGIN OF THE TAYLORSVILLE MESOZOIC RIFT BASIN, VA

Pseudotachylites in brittle fault zones result from frictional melting that can occur during high-velocity slip events (i.e., earthquakes). In as much as these melts commonly contain wt% K₂O, they are amenable to ⁴⁰Ar/³⁹Ar dating and provide an opportunity to directly date brittle faulting and ancient seismicity. However, interpretation of Ar isotopic data from pseudotachylites is complicated by their very fine-grained to glassy character, the entrainment of relict grains (and their inherited Ar), and their susceptibility to alteration.

Here we present microscopic characterization and Ar isotopic data from pseudotachylite exposed in the damage zone of the bounding normal fault of the Taylorsville basin, a Mesozoic half graben in east central Virginia. Gneisses in the footwall, exposed along the North Anna River, are cut by mm- to cm-wide veinlets of pseudotachylite. These veinlets were studied by optical and electron microscopy, electron microprobe, microXRD, and Raman spectroscopy. The data demonstrate that abundant entrained quartz and plagioclase are present in the veinlets, but K-feldspar is rare. At ~100 µm², the pseudotachylite matrix has a nearly uniform trachyandesite composition and is dominated by very fine-grained plagioclase and biotite. MicroXRD of the veinlet resolves no amorphous material. However, spot Raman analyses show a broad alumino-silicate peak at ~1000 cm^-1 suggesting some glassy material is present.

Because sample characterization showed that the pseudotachylite is largely unaltered and that K-feldspar entrainment from wall-rock is minor, the samples were amenable for ⁴⁰Ar/³⁹Ar analysis. Sub-mm diameter microcores were drilled from thick sections of pseudotachylite. Micro-coring is advantageous over laser ablation because it preserves the microtextural context and allows for step-heating of the samples. Step heating experiments on 5 microcores yield ⁴⁰Ar/³⁹Ar spectra that have flat portions comprising ~80% of the ³⁹Ar_K and Ca/K ratios consistent with the electron microprobe-determined composition of the pseudotachylite matrix. These steps yield weighted average ages of 218-210 Ma which are interpreted as the crystallization age of biotite from the pseudotachylite melt and a time of seismicity on this basin-bounding fault.