HORIZONTAL TRANSPORT OF PICTURE GORGE BASALT MAGMA THROUGH MONUMENT DIKE SWARM DETERMINED BY ANISOTROPY OF MAGNETIC SUSCEPTIBILITY

Flood basalts of the Miocene Columbia River Basalt Group (CRBG) cover 210,000 km² of Washington, Oregon, and Idaho. The origin and source of CRBG melts are debated – widely spaced feeder dike swarms project toward a hypothetical source area near the SE Oregon-Idaho border. CRBG ages and dike swarms young to the north, suggesting magma migration away from this central source. A similar scenario was documented using anisotropy of magnetic susceptibility (AMS) measurements for the Precambrian Mackenzie dike swarm, with lateral flow over 1000s of kilometers. Here we use AMS measurements to directly track magma flow in the Monument dike swarm (MDS), the feeder dikes of the Picture Gorge Basalts (PGB). The PGB is a smaller member of the main phase of CRBG eruption, erupting coincidently with the voluminous Grande Ronde Basalt (~16.3 Mya).

Magnetic fabric measurements were made on 250 oriented paleomagnetic specimens, subsampled from 118 samples that were collected from 14 dikes of the MDS. Sampled dikes varied in width from ~20cm to ~40m. AMS is the directionally dependent response of a specimen to an induced magnetization and is a function of the magnetic mineral properties (e.g., size, shape, domain state, orientation). Magma flowing through dikes has been shown to acquire an anisotropic magnetic fabric with a minimum axis (K₃) perpendicular to the wall and maximum axis (K₁) aligned in the direction of flow. Inverse fabric, with the K₁ axis perpendicular to dike walls, can occur when magnetic minerals are of single domain size or when flow in the dike is “forceful injection” rather than “free flow”.

We see clear evidence for lateral flow in 9 of the 14 dikes. The mean K₁ axis direction is sub-horizontal and aligned NW-SE, which is also the trend of the dikes. One dike has an isotropic AMS ellipsoid. Four dikes show evidence of inverse fabric with the K₃ axis aligned sub-horizontally NW-SE; further testing is needed to determine the cause. Where it was possible, we sampled profiles across the dikes to investigate the polarity of flow from imbrication near dike margins, but the results are inconclusive. We conclude that the magma source for the PGB was distal, with magma transport via horizontal flow feeding eruptions away from the hypothesized source to the southeast.