KINEMATICS OF THERMALLY-INDUCED NEOGENE CROSS NORMAL FAULT SYSTEM AROUND THE SNAKE RIVER PLAIN, SE IDAHO AND SW MONTANA

Late Cenozoic Basin and Range (BR) normal fault blocks have continuously been deforming over the past 16.6 Ma by a younger and diachronous Neogene cross normal fault (CF) system around the Snake River Plain (SRP) in SW MT and SE ID. Data about the CF traces were acquired from a combination of USGS fault databases and geological maps at scales of 1:100,000, 1:250,000, and 1:1000, 000. The azimuth of the linear directional mean (LDM) of the set of CF traces were measured (in ArcGIS 10) in five temporal domains (T₁-T₅) classified based on their position relative to the successive centers of eruption of known age along the SRP. The LDMs of the CF system in each temporal domain were also spatially determined in the southern part (domain S₁), central part (domain S₂), and northern part (domain S₃) of the SRP. The LDM in each spatial domain approximates the orientation of the intermediate principal compressive axis (s₂), so it allowed us to estimate the orientation of the horizontal component of extension for normal faulting. Most of the cross fault LDMs in the latest temporal domains are at a high angle to the trend of the eastern SRP. The trajectory (form line) of the LDMs for the three spatial domains of each temporal domain has an asymmetric, sub-parabolic form, with its apex centered along the axis of the SRP. The aggregate pattern of the sub-parabolic LDM form lines for all temporal domains, especially for the T₃-T₅ domains, is similar to the reported parabolic distribution of earthquake epicenters along active normal faults around the Yellowstone hotspot (YHS). This similarity indicates that the reported epicenters lie along the traces of the cross normal faults which are studied by us. The spatio-temporal distribution and similarity of the sub-parabolic LDM form lines for different ages, the position of their apices on the centers of eruption along the SRP, and the inferred extension directions for each time of eruption, suggest that the CFs progressively formed due to the thermal regime of the hotspot as it migrated to the NE due to an extensional field in front of the hot spot, which was distributed radially across the sub-parabolic LDM trajectory during each thermal event.