Abstract Title: Re-examining the transition from the Basin and Range Tectonic 
Province to the Middle Rocky Mountains Tectonic Province

Abstract Author(s): Pankow, Kris L. (University of Utah) - Bruhn, Ronald L. 
(University of Utah)
  
Abstract: The Wasatch fault, a 380-km-long northerly striking normal fault, is 
commonly thought to mark the eastern boundary of the Basin and Range tectonic 
province. In the south this boundary is a division between the Basin and Range 
and the Colorado Plateau and in the north between the Basin and Range and the 
Middle Rocky Mountains. Proximal to the Wasatch fault there is also a 
transition in crustal thickness and changes in surface heat flow. It thus seems 
quite reasonable that this fault delineates the tectonic boundary. However, a 
striking feature of the Wasatch fault, especially on the northern half, is the 
lack of background microseismicity. In fact, an examination of a Utah 
seismicity map reveals that the predominant band of seismicity in northern Utah 
is 15-40 km east of the Wasatch fault. Previous studies have hypothesized that 
this seismicity occurs along old thrust belt structures, while other studies 
have hypothesized that the earthquakes are the result of flexure. A better 
understanding of this microseismicity could provide important information for 
understanding the transition from the tectonically active Great Basin to the 
more stable North American craton. Preliminary results of a detailed study of 
this microseismicity using waveform comparisons, relative relocations, and 
focal mechanisms will be presented. These preliminary results will be 
interpreted with respect to known thrust belt structures and regional 
geophysical information. Ultimately our goal is to test the various hypotheses 
related to the source of the microseismicity and better characterize this 
important transition zone.