PALEOMAGNETISM OF COLUMBIA RIVER BASALT GROUP DIKES: UTILITY AND APPLICATION TO LARGE IGNEOUS PROVINCE PLUMBING SYSTEMS

The Columbia River Basalt Group (CRBG) includes thick sequences of erupted basalt flows, as well as its plumbing system, with thousands of feeder dikes exposed. This contrasts with older large igneous providences of which the erosional remnants are usually dike swarms cutting across ancient cratons. Due to this preservation bias, only paleomagnetic and geometric data from mafic dike swarms—and not the associated basalt flows—can be typically used in Precambrian paleogeographic reconstructions.

In contrast, paleomagnetic studies of the CRBG have primarily focused on basalt flow magnetostratigraphy, with few studies concerning the abundant dike swarms. Here we present initial results from a detailed paleomagnetic study of 15 dikes of the Monument swarm. These dikes fed the Picture Gorge Basalt (PGB), a small but representative formation within the CRBG. Our data, used in concert with geologic and geometric information from the dikes themselves, can track deformation in the same way as paleomagnetic datasets from the PGB flows. The paleomagnetic directional data from the dike swarm are directly comparable to those of the PGB basalt stratigraphy; the dike swarm records an identical vertical-axis rotation as seen in the basalt flows. We also examine extant paleomagnetic data from other CRBG dikes and discuss their implications for studies for paleogeographic studies of Precambrian mafic dike swarms in the more distant past. We conclude that although intrusion-only studies likely underestimate geomagnetic field variability they provide robust paleogeographic constraints.