TECTONIC FABRIC OF EASTERN NORTH AMERICA

The tectonic fabric of basement rocks of eastern North America records successive episodes of assembly and breakup, including the most recent two complete Wilson cycles since ~1.3 Ga. The older (>1.3 Ga) provinces/terranes reflect accretion, overprinting, and inheritance, culminating with pluton complexes and layered volcanic rocks of the Granite-Rhyolite province.

The Grenville province (1.3–1.0 Ga) records assembly of supercontinent Rodinia and forms the weld between conjugate cratons, encompassing protoliths of proto-Laurentia and Amazonia. The probable suture is a prominent intra-Grenville shear zone/transform fault. On the Laurentian side, accretionary fabrics extend to the leading tectonite Grenville Front.

During breakup of Rodinia and opening of Iapetus (765–530 Ma), the rifted continental margin cut across Grenville fabrics, leaving some Amazonian elements attached to Laurentia. Transform offsets of the rift outlined promontories and embayments of the continental margin. Rift-related fabrics include intracratonic rift-parallel and transform-parallel fault systems overprinted on older basement rocks inboard from the rifted continental margin of Laurentia.

Closing of Iapetus and assembly of Pangaea was a protracted process (~470–285 Ma) of accretion of arc and continental terranes, continental collision, and transpression. Manifestations in the foreland are primarily in the upper crust in a shallow thrust belt and foreland basins; however, these events contributed to thickening of the crust. The interior of the orogen consists of crustal-scale accreted elements, along with deep accretionary fabrics and shear zones.

Breakup of Pangaea and opening of the Atlantic (since ~210 Ma) formed a rifted continental margin with transform offsets, many at sites inherited from older fabrics. Intracratonic fault systems show that extensional fabrics extended into the new North American continent.

With the myriad fabrics in North American basement rocks, the distribution of modern seismicity shows no simple association with any specific older structure. Some examples (East Tennessee, New Madrid, Charlevoix) suggest, however, that locally damaged (dilated) crust is associated with various tectonic settings of older structural fabrics and may be the cause of concentration of seismicity.