TRANSITIONS FROM DUCTILE TO BRITTLE DEFORMATION AND THE CONSEQUENTIAL EVOLUTION OF MESOZOIC BASINS FROM PALEOZOIC SHEAR ZONES

Mesozoic rift basins in the Piedmont of the Southern Appalachian orogen are typically superimposed on regionally discrete shear zones. The shear zones contain tectonite fabrics that record a range of conditions, but most commonly ductile shear and the production of mylonites whose specific properties reflect the protolith rheology. Depending on the metamorphic grade of wall rocks adjacent to the shear zones, ductile deformation was coincident with retrograde metamorphism typically downward into greenschist facies assemblages. The most intense alteration of this kind is in those parts of the shear zones that are in close proximity to Mesozoic basins.

The ductile shear zones provide three forms of mechanical control on the nucleation and evolution of rift basins. First, the mylonites were last recrystallized near the ductile to brittle transition and often contain a pervasive laminar fabric. Small kink and chevron folds of the mylonite foliation suggest that the foliation is weak enough to accommodate slip in the formation of these kinds of folds. Larger folds have asymmetries and senses of vergence indicating that at least part of the extensional strain leading to the opening of the Mesozoic basins was guided by the older foliation. Diabase sills of probably Jurassic age intruded along the foliation as well. Second, because the shear zones contain mechanically weaker rheologies than the higher metamorphic grade crystalline wall rocks the shear zones acted as stress guides for later mostly brittle deformation. Third, some of the shear zones juxtapose rocks of different composition and strength, enhancing the mechanical contrasts across the shear zones.

The conditions of deformation in the ductile shear zones approach the likely conditions of deformation in the initiation of the Mesozoic basins, and it is compelling to think of the relationships between the shear zones and basins as consequential instead of subsequential. Though clasts of strongly foliated mylonite in basin alluvial deposits clearly show that parts of the shear zones were exhumed at the time of deposition, it is still possible that the latter, more brittle structural elements of the shear zones are part of a rapid transition across the brittle to ductile transition that bridged the time interval between latest Paleozoic orogen-parallel shear and early Mesozoic extension.