PLEISTOCENE FAULTING AND SEDIMENTATION IN THE VICINITY OF THE 2020 SPARTA, NORTH CAROLINA EARTHQUAKE REVEALED BY COSMOGENIC 26AL/10BE BURIAL DATING

The Mw 5.1 earthquake that struck Sparta, North Carolina on August 9, 2020, motivated detailed mapping of ruptures and geology in the area. Field mapping over the past two years has revealed numerous exposed faults, as well as preserved fluvial and colluvial gravels that blanket some fault surfaces. The existence of these gravel deposits raises questions about the interplay between tectonics, climate, and sedimentation in this seismically active landscape.

We have sampled five deposits in Sparta and the surrounding area for cosmogenic ²⁶Al/¹⁰Be isochron burial dating, with the goal of obtaining depositional ages and paleo-erosion rates. Thus far, we have measured ²⁶Al/¹⁰Be for three sampling sites: two gravel accumulations along Bledsoe Creek in Sparta, and one buried terrace 3 km ENE of Jefferson, North Carolina. All deposits are located along tributaries of the New River.

The Reston Cosmogenic Nuclide Laboratory, a new USGS facility for extraction of cosmogenic ²⁶Al and ¹⁰Be from quartz, processed the samples. Quartzite cobbles and quartz-rich sand were cleaned using conventional techniques, assayed via inductively coupled plasma - optical emission spectrometry, and prepared for accelerator mass spectrometry measurement at the Purdue Rare Isotope Measurement (PRIME) Lab. Measurements revealed early to middle Pleistocene ages for the deposits. The terrace near Jefferson was buried at 1.01 ± 0.09 Ma, whereas the gravels near Sparta were deposited at 0.49 ± 0.14 Ma and 1.85 ± 1.47 Ma; the imprecision of the latter age is likely a consequence of shallow burial and high postburial production. The two former deposits respectively precede major glaciations at ~0.8 Ma and ~0.2 Ma. Paleo-erosion rates derived from the sand fractions ranged from 9-17 m/My, consistent with modern erosion rates measured from cosmogenic ¹⁰Be elsewhere in the Blue Ridge Mountains. Paleo-erosion rates from mixed gravels (<5 m/My) are likewise consistent with Pleistocene paleo-erosion rates from New River cave deposits near Pearisburg, Virginia. These preliminary results suggest that despite faulting and recent seismicity near Sparta, regional erosion rates are similar to the rest of the Blue Ridge. We are currently working to date additional gravels from the Sparta area and further improve the surficial geochronology.