PLACING RECENT SEISMICITY AT THE SAVANNAH RIVER SITE (SC) IN THE CONTEXT OF REPEATED PHANEROZOIC FAULTING

Fifteen instrumentally located earthquakes since 1974 lie within a fifty km radius of SRS. The most recent activity includes five small earthquakes that occurred 10/01 in the north central portion of SRS. Activity comprised a main, felt event, with duration magnitude of 2.5 and four aftershocks of Md < 1.5. Plotting hypocenters of these five events on SRS seismic reflection line 4 shows them to occur between shotpoints 200-530 (NE along strike from wells GCB-2, MMP-4, P30) at depths 3 - 5 km (or ca. 1.51 - 2 s TWTT). This is nearly coincident with the fault contact between very low grade Crackerneck Metavolcanic Complex and intermediate P/T Deep Rock Metaigneous Complex. To the southeast of this most recent seismicity metavolcanic rocks (ca. 619 Ma–626 Ma),beneath 100-400m of coastal plain, preserve two late Paleozoic ductile shear zones that bound an overturned nappe limb. These highly strained rocks controlled the location of the Dunbarton Tr basin border fault and later fluid flow and Tertiary reactivation. Ca. 220 +/- 5 Ma, a K2O + SiO2 metasomatic event ("pinking") affected SRS basement. A "greening" event flushed highly reducing fluids through basement, Tr fanglomerates and rocks interpreted to be as young as Santonian (Cape Fear Fm). The last event of a consistent Cretaceous and younger vein paragenesis is a near vertical strike-normal zeolite set. A K-Ar age of euhedral, 2mm x 7mm zeolite grains of the last set yields an age of 22.9 +/- 1.5 Ma, interpreted to be a minimum age. More than 30 pseudotachylytes were identified in core in the metavolcanic rocks and are preferentially localized along Alleghanian-age chloritic fractures dipping less steeply than foliation that have been “pinked.” Pseudotachylyte can be shown to consistently post-date mineralized fractures. Thus, pseudotachylytes from SRS core are likely no older than Mesozoic, and many are probably Tertiary in age. The Pen Branch fault is "exposed" in approximately 250 m of PBF-7 basement core between 1375’-2168’. Most recent motion on PBF must postdate strike-normal zeolites, because the zone cross-cuts all mineralized fractures. Three earthquakes from 6/85 (Md=2.6) 8/88 (ML=2.0) and 5/97 (Md=2.5) have hypocenters that plot within the Tinker Creek nappe in crystalline basement.