Abstract Title: What's Moving In The Basin-Range!

Abstract Author(s): R. Smith, W. Chang, G. Waite, C. Puskas (University of 
Utah) and A. Lowry (University of Colorado)  

Abstract: A multidisciplinary approach is employed to evaluate active tectonism 
of the Basin-Range (BR) and its transition into the stable plate along the 
1300-km long Intermountain seismic belt (ISB). This intraplate regime is 
characterized by shallow normal-faulting earthquakes, active volcanism, high 
heat flow and a thin crust. This zone also coincides with an abrupt change in 
the elastic thickness from < 15 km in the BR to as large as 60+ km in the 
adjacent plate interior as well as notable decrease in viscosity into the BR. 
Complicating the BR extensional stress regime are the effects North America 
plate interaction with the Yellowstone hotspot. Contemporary kinematics, 
defined by GPS measurements, reveal: 1) general NE extension of ~4 mm/yr. 
across the Yellowstone volcanic field that diminishes to 2.5 mm/yr along the 
central SRP, but still a major component of BR extension, 2) a notable change 
to E extension southward entire length of the Intermountain region, and 3) 
rotating to NW across the western BR. Moreover, the eastern BR has experienced 
two of the largest historic earthquakes, the M 7.5 1959 Hebgen Lake, MT, and 
the 1983 M7.3 Borah Peak, ID, events. These earthquakes provide a working model 
for extensional regime nucleation that includes slip on moderate to steeply 
dipping, planar faults at mid-crustal depths near the brittle-crustal 
transition. This model however, points out an unresolved difference for 
shallow, low-angle to listric normal faults restricted to the upper 7 km of the 
crust revealed from seismic reflection data. Campaign GPS and continuous GPS 
data provide the first determination of contemporary deformation of the region 
including concentrated strain across the eastern BR. Non-linear models of fault 
dislocation from GPS data reveal loading rates beneath the seismogenic layer of 
3 to 5 mm/yr, notably greater than the fault slip rates deduced from the 
Holocene fault record on the Wasatch Front. However GPS data across the Hebgen 
Lake fault show continued extension of up to ~3 mm/yr that has been modeled for 
rheological properties showing a strong upper crust underlain by weak lower 
crust, similar to depths of the modeled brittle-ductile transition. Using the 
rheological models we have modeled the time-dependent deformation of paleo- and 
large-historic earthquakes. These results suggest that post seismic deformation 
decays over a few hundred years with ~50 yr. Maxwell times. A preliminary 
unified kinematic model of the region is being constructed with input of GPS 
and fault slip rates of the entire region in a 3D finite-element model. It 
reveals a fan-shaped, clockwise opening across the northern BR from a potential 
energy high over Yellowstone, rotating to generally E-W to NW extension across 
the north and western Basin-Range. On a regional scale, this part of the 
western plate interior exhibits dynamic topography dominated by buoyancy 
variations of the lithosphere and contributions reflecting the combined effects 
of Laramide crustal-shortening, Late Tertiary extension, crustal buoyancy and 
thermal boundary processes. The largest buoyancy anomaly, > 1000 km in radius, 
extends across the northern BR and may have formed coincidently with the latest 
stage of increased extension at ~17 My.