16 Geologic Maps, Geophysical Maps, Digital Geologic Maps, and Derivatives from Geologic and Geophysical Maps (Posters)

Thursday, 24 March 2011
1:30 PM - 5:30 PM
Exhibit Hall

DETAILED GEOLOGIC MAPS OF TWO SITES SOUTH OF DANDRIDGE, TENNESSEE, RECORD EVIDENCE OF POLYPHASE PALEOSEISMIC ACTIVITY IN THE EAST TENNESSEE SEISMIC ZONE

Paper number: 16-1
Program: Theme Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

The East Tennessee seismic zone (ETSZ) in the southern Appalachians is the second most active intraplate region east of the U.S. Rocky Mountains. This ~50 km-wide, 300 km-long zone of seismicity extends from NE Alabama and NW Georgia to just NE of Knoxville, Tennessee, producing earthquakes at depths of 5-26 km within the basement below the major Paleozoic thrust sheets. Although the ETSZ has not produced historical earthquakes of M>5, others have suggested that it may be capable of generating an “infrequent” M~7.5 event. Small faults and numerous bleached-clay-filled fractures discovered in late Quaternary French Broad River terraces (optically stimulated luminescence ages of terraces range from several tens to >100 ka) along Douglas Reservoir led to a need to grade and map (at 1 in = 5 ft) two small areas E and W of the TN 92 bridge S of Dandridge, Tennessee. The site W of the bridge yielded less information, but still revealed at least three sets of crosscutting fractures (dominant orientations 045, 065, 085, 305; 553 measurements from map) that terminate up-slope against the base of an overlying colluvium. The E site revealed numerous fractures (dominant orientations 055, 305, 335; 701 measurements) and a fault with ~20 cm displacement. Moreover, several bedrock shale “boils” are present that are cut by younger, red clay-filled fractures. Few fracture sets in Quaternary sediments parallel those measured in Tennessee Valley and Ridge and Blue Ridge bedrock, but these fractures do not relate well to the present-day 070 orientation of maximum principal stress. The small faults discovered here and elsewhere around Douglas Reservoir all have approximately the same displacement; comparison with estimates of earthquake magnitude from active faults in bedrock suggests the earthquakes that produced these faults were M>6. They have a recurrence history spanning thousands of years.