JUXTAPOSED MESOZOIC DIABASE DIKES AND SILICEOUS CATACLASITE FAULT ZONES IN THE CAROLINAS AND THE MECHANICS OF DIKE EMPLACEMENT

Two crossing Mesozoic (190–170 Ma) diabase dikes sets exist in the Carolinas Piedmont and Coastal Plain. They formed coevally with small displacement siliceous cataclasite fault zones. The dominant NW-trending regional diabase dike set related to breakup of Pangaea was known decades ago. The other diabase dike set fans northward in a ~10° arc from a point between Charleston and Georgetown, SC, and crosscuts the regional dike set. Only minor differences exist in the geochemistry of the dike sets—both are olivine normative with similar major, minor, and trace element compositions—probably sampling the same mantle lithosphere. The regional siliceous cataclasite fault zones formed at about the same time, with ~E–W and lesser ~N–S orientations, and bisect the regional dike orientations. Dike thicknesses range from one to >300 m, with most ranging 5 to 25 m thick. Only the thickest dike segments have appreciable contact aureoles, which, along with a lack of geochemical crustal contamination, favors short magma residence time in the crust and rapid crystallization. The tabular shape and surface area of dikes requires almost hypervelocity crack propagation through the crust by “magma fracturing” to prevent rapid heat transfer and crystallization before the dikes reach the upper crust. The regional dike pattern has been explained by crack propagation from a linear central region during the initial breakup of Pangaea (e.g., Paul May, 1971, May GSAB), and the siliceous cataclasite faults occupy shear planes in a strain ellipsoid accommodating the diabase dikes along the YZ plane. The divergent diabase dike set has been attributed to a shallow mantle plume beneath the divergent point that would have had to generate an asymmetric extensional stress field. Instead of a plume, such a localized stress field might be generated by a short-term crustal or lithospheric perturbation during opening of the Atlantic.