Synergy of Inherited Structures and Modern Processes in the Eastern Tennessee Seismic Zone

Session: Seismicity and Tectonics of Stable Continental Interiors
Type: Oral
Date: 4/30/2020
Time: 02:15 PM
Room: 240
Description: 

As with most intraplate seismic zones, the cause of high earthquake rates in Eastern Tennessee (ETSZ) is unknown. Here, we begin by cataloging aspects of the ETSZ that differ from other parts of the central/eastern United States (CEUS) and then explore links between these anomalies and seismicity. During Proterozoic shear, a releasing bend in the southern ETSZ created heavily fractured crust atop the subsequent lithospheric suture. There, the receiver function Moho is faint or absent, and the ETSZ as a whole features low-velocity, conductive middle/lower crust and the lowest-density lower crust in the CEUS. Buoyancy overcompensates topography, manifest in highly negative isostatic residual gravity. Some tomograms suggest lower lithospheric removal and resultant upwelling along the suture. Miocene drainage reorganization triggered a rapid pulse of erosion now sweeping through the ETSZ. Most critically, focal mechanisms and stress inversions document oblique extension that is unique in eastern North America. We create a 3D model of lithospheric density and finite-element model of associated stress, then solve for the tectonic stress that—when summed therewith—best reproduces stress directions and faulting styles across the CEUS. Stress due to buoyant lower crust overwhelms regional tectonic compression, accounts for the anomalous extension and elevates net stress, focusing strain. We propose a unified explanation for these phenomena. Mantle-derived fluids (from any of several periods of subduction) preferentially flux along the inherited lithospheric-scale fractures and then the pervasive crustal fault system. Hydration-induced retrogression of garnet-bearing lower crust to less dense assemblages produces conductive, slow, buoyant lower crust. Increased buoyancy excites uplift, erosion and further rebound, increasing flexural tension and relieving confining stress on faults. In this conception, the synergy of features and process responsible for seismicity is unique to the ETSZ, such that future seismicity may remain largely confined to the same region.

Presenting Author: Will Levandowski

Authors