3D-MODELING AND MAGNETIC (AMS) ANALYSIS OF MESOZOIC DIABASE DIKES IN NORTH CAROLINA

Diabase dikes that formed during Triassic-Jurassic breakup of Pangea are well-exposed in quarries in eastern North Carolina. In a quarry north of Rocky Mount, NC, steeply dipping, north-striking dikes up to 10 m thick cut ~300 Ma granite of the Rolesville batholith. We are conducting 3D mapping and anisotropy of magnetic susceptibility (AMS) analyses in an effort to determine magma flow direction and dike opening directions.

Dikes in the quarry are typically composite, consisting of individual dikelets that range up to ~2 m in width. We have constructed 3D models and orthoimagery of the dike network using drone imagery and Pix4D software; additionally, archived surveys done by the quarry operators allow us to reconstruct parts of the network that have been quarried out. Preliminary AMS data from four dike samples yield an anisotropic fabric with an average mean susceptibility (K_mean) of 0.023 SI and corrected anisotropy degree (P) of 1.035. Tightly clustered susceptibility axes and a plot of shape tensor (T) against P reveals a moderately-oblate fabric with an average foliation oriented 007/60. Dike walls strike ~350, so the AMS fabric is rotated ~20° clockwise. AMS analyses of two granite wall rock samples yielded a different fabric, with an average shape tensor of -0.3 and anisotropy degree of 1.166, corresponding to a moderately prolate fabric.

The origin of the oblique magnetic fabric is puzzling, as foliation formed by mineral alignment during magma flow should be parallel to dike walls. Therefore, other tectonic stresses likely dampened or skewed the original flow fabric. The observed fabric is parallel to the much older foliation fabric in the metamorphic wall rocks of the Rolesville batholith, but this relation is likely coincidental. It is possible that the oblique fabric formed during oblique opening of the dikes, as did dikes of the Jurassic Independence swarm in California. If so, this oblique opening would imply that a right-lateral component was included in the tensional stresses that opened the dikes. Further AMS and geometrical studies will be used to test this hypothesis.