STRUCTURAL CONTROLS OF INTRAPLATE EARTHQUAKES IN THE EASTERN UNITED STATES

The premise - that intraplate seismicity occurs along pre-existing post-orogenic brittle faults formed during extensional events – permits distinguishing three large intraplate-earthquake domains (accreted terranes of the Appalachians, Grenville, and mid-continent) in the eastern United States separated by sealed metamorphic boundaries. Younger migrating hotspots and failed rifts provide connecting zones across sealed metamorphic boundaries. Fracture studies within southern Appalachian accreted terranes, Triassic rift basins, and overlapping Cretaceous-Cenozoic passive-margin strata (southern Atlantic Coastal Plain) show that recurrent brittle faulting:

• is along post-orogenic fracture-sets formed during failed-to-successful Mesozoic rifting;
• is not related to brittle reactivation of ductile shear zones or metamorphic fabrics;
• is largely confined to Appalachian accreted crystalline terranes separated by sealed terrane boundaries;
• is within a temporal sequence of fracture sets with a hierarchical ordering and scaling of recurrently active fault zones with older, linked-sets forming more through-going fault zones which bound large polygonal crustal blocks.

Grenvillian crystalline terranes (1.2-0.9 Ga), accreted during amalgamation of the Rodinian supercontinent, contain post-orogenic brittle fracture sets initiated during the Rigolet-phase (1.02-0.9 Ga) shift from contraction to NW-SE-extension with development of core complexes (Adirondacks); followed by protracted cooling; continued NW-SE-extension; and passage of a hotspot (750-600 Ma) during the breakup of Rodinia and opening of the Iapetus Ocean (565 Ma). Mid-continent Archean-Mesoproterozoic orogenic belts are cut by brittle fracture sets related to: failed rifting events (1.1 Ga Mid-Continent rift; Cambrian Southern Oklahoma aulacogen, Reelfoot rift, Rough Creek graben); successful Triassic rifting (Gulf of Mexico); and mid-Cretaceous passage of the Bermuda hotspot.