THE SUWANNEE SUTURE AND TERMINAL ALLEGHANIAN COLLISION: REINTERPRETATION FROM INTEGRATED ANALYSES OF POTENTIAL FIELD DATA

The basement rocks concealed beneath sediments of the southeastern Atlantic Coastal Plain record the Late Paleozoic interaction between Laurentia and Gondwana as reflected in regional-scale structures, metamorphism, and magmatism. Southeast of the Fall Line, legacy aeromagnetic data have been used to define the suspect Charleston terrane, and the Gondwanan Suwannee terrane, yet despite the longevity (1980s) of these tectonic subdivisions, little subsurface data exists to differentiate these terranes or locate the boundaries between them. A boundary of particularly interest is the Higgins and Zietz Line (HZ Line), a magnetic boundary which has been interpreted as the Alleghanian suture and also the transcurrent Carolina-Mississippi fault.

Tilt derivative maps of frequency filtered total field magnetic data shed new light on the lithospheric configuration in the Late Paleozoic by better resolving terrane boundaries, and better defining the nature and extent of Alleghanian deformation. Regional deformation and metamorphism is reflected in a distinctive magnetic grain visible on derivative maps that is continuous from the frontal Blue Ridge thrust across the HZ Line, and terminates in a previously unrecognized ENE-striking tectonic boundary in South Carolina and eastern Georgia. This 200-km-long boundary projects near the landward limit of the Gondwanan Suwannee Basin offshore, and is interpreted as the northward extension of the Suwannee (Alleghanian) suture. The absence of any magnetic boundaries on the eastern side of the suture is consistent with the Charleston and Suwannee terranes amalgamating prior to Alleghanian deformation and accreting to Laurentia together.

Integrated potential field forward modeling and structural modeling suggest that the relatively undeformed sediments of the Suwannee Basin can be explained by the ‘soft landing’ of the preserved basin at the trailing edge of the Gondwanan mobile margin along a low-angle ductile detachment (Appalachian decollement) that extends to depths near the base of the crust, and does not require displacement along a continent-scale strike-slip fault (Carolina-Mississippi). Modeling also suggests that the Suwannee suture was reactivated during Mesozoic extension, locally forming a major border fault of the South Georgia rift, and that the Alleghanian detachment may have been exploited during Atlantic rifting.