MESOPROTEROZOIC TO CENOZOIC FAULTS AND FAULT ZONES ASSOCIATED WITH THE COMPLEX TECTONIC HISTORY OF THE APPALACHIAN BLUE RIDGE

Mesoproterozoic to Paleozoic rocks in the Blue Ridge are cut by faults and fault zones (FFZs) that formed during extension and contraction over a span ca. 1 billion years, and under various tectonic loads. FFZs are important structural elements that help define the regional tectonic history. FFZs may be discrete brittle faults or broad zones of anastomosing ductile deformation, and may be reactivated, folded, or cut by younger faults. Named FFZs terminate and do not link along strike, thus these conceptual “lines” may or may not mark an important boundary. Some FFZs originate as detachments along beds above unconformities and place young rocks on old rocks, while others are defined by truncated isograds, or are inferred by discontinuities of ⁴⁰Ar/³⁹Ar and fission track ages. Out of sequence emplacement may be complex rather than simply stacked sheets. Protracted deformation also obscures many FFZs that were associated with earlier discrete tectonic events.

Evidence of brittle normal faults that accompanied extension are associated with: 1) ca. 780-750 Ma rift basins that contain bi-modal volcanic deposits; 2) linear batholiths of ca. 750-700 Ma granitoids; 3) half-grabens that contain seismites that are older than ca. 575 Ma bi-modal volcanism associated with rifting; 4) a submarine slide in ca. 540 Ma marine clastic deposits; 5) a post-ca. 510 Ma half-graben of shelf carbonate rocks juxtaposed on ca. 575 Ma basalt; 6) growth faults associated with 220-200 Ma basin deposits that also cut ca. 200 Ma diabase, and 7) Cenozoic uplift along older FFZs.

FFZs associated with orogenies are more complex. Some contraction predates metamorphism and others exhibit syn- and post-metamorphic ductile deformation. Examples include: 1) northwest-striking foliated units associated with the Grenville orogeny; 2) FFZs interpreted to be “sutures” bounding different “terranes” may juxtapose the remains of a slope-rise sequence and oceanic rocks that are younger than the older continental margin rocks; bodies of mafic rocks could constitute sedimentary blocks and tectonic mélange that post-dates crystallization and emplacement; and 3) Devonian-Mississippian FFZs that are broad zones of mylonitic foliation that were folded above Pennsylvanian-Permian duplex.