RECENT GPS STUDIES ENHANCE THE MODEL OF THE INTERMOUNTAIN SEISMIC BELT BEING EVIDENCE FOR THE EAST PACIFIC RISE SPREADING CENTER OPERATING UNDER THE NORTH AMERICAN PLATE

The Intermountain Seismic Belt (ISB)/East Pacific Rise (EPR) Spreading Center Plate Tectonic Model (Model) [originally the “Wasatch Line (Neogene)/EPR Model” at Northwest Geology, Vol. 41, July 2012] interprets the ISB to be evidence that a currently operating, subsialic northern portion of the EPR spreading center is generating southwesterly and northeasterly vectoring divergent “oceanic” basaltic plates that move horizontally immediately under North American Plate (NAP). The Model suggests the subsialic EPR began operation about 14 Ma and has dominated US Cordilleran geology since that time. Baird (2012) derived and described the model primarily from topographic, geologic and structural evidence. However, recent GPS studies of the central ISB, e.g., Payne et al. (2012) and Kreemer et al. (2010) offer independent support for key concepts of ISB/EPR model but also require the modification of the model to interpret the Idaho seismic zone as integral to the main ISB, per Smith & Arabasz, 1991, not as an independent outlying seismic feature.

Collectively, the GPS studies describe connected NAP regional strike-slip and extensional surface movements in paired sequences that may be characterized as mimicking “mid-ocean” ridge spreading centers/transform structures and plate vector patterns. Integration of the GPS studies and model suggests the following NAP structures are evidence for subsialic ISB/EPR alternating transform faults or spreading center segments, as indicated: Montana ISB spreading center (north of the trans-Challis fault system [TCF]); TCF transform (sinistral); Idaho Pioneer/White Clouds spreading center; Centennial transform (dextral); Yellowstone spreading center; Eastern Snake River Plain (south edge) transform (sinistral); Great Salt Lake spreading center; Pahranagat Shear/Southern Nevada transform (sinistral); and, Kaibab Plateau spreading center. The ISB/EPR Model provides a mechanism to account for the otherwise anomalous pattern of alternating regional strike-slip and regional extensional movement identified by GPS studies.