MORE THAN ONE WAY TO SHEAR: ACCOMMODATING VARIABLE EXTENSION IN THE NORTHERN BASIN AND RANGE AND EASTERN SNAKE RIVER PLAIN

The eastern Snake River Plain is a critical point at which to study the Yellowstone volcanic system. Dextral shear has been proposed between the Centennial tectonic belt and the eastern Snake River Plain, yet no through-going strike-slip faults have been identified. Deformation features within a thick conglomerate bounding the eastern Snake River Plain highlight a distributed transtensional shear zone likely dating back to the Pliocene. Strain is distributed across numerous small-scale, high-angle conjugate transtensional faults which loosely lie sub-parallel to the orientation of maximum principal stress (σ₁). Anomalously large pressure-solution pits have a preferred sub-horizontal NE/SW orientation, interpreted as representing the σ₁ orientation. Distributed strain occurs in a wide (>20 km) yet discrete primary deformation zone, providing field evidence for the Centennial shear zone hypothesis. Kinematic disparities between ductile extension beneath the seismogenic crust of the eastern Snake River Plain and brittle extension in the seismically active Centennial tectonic belt are the preferred driver of dextral shear. Disparities in extension magnitude within the Basin and Range are accommodated as oblique slip along preexisting normal faults within the northern Basin and Range between the eastern Snake River Plain and the Lewis and Clark fault zone. Strain is distributed along numerous small scale, high-angle transtensional faults in weak or relatively undeformed strata. Oblique slip on preexisting normal faults accommodates shear where major basement weaknesses are present. We propose a preliminary kinematic model for the formation of the eastern Snake River Plain and the resultant Centennial shear zone characterized by (1) initial NW/SE sinistral shear, (2) subsequent NE/SW dextral transtensional shear along the boundaries of the plain and finally (3) coupled low magnitude NE/SW extension between the eastern Snake River Plain and the surrounding Basin and Range. This major transtensional system, culminating at the modern Yellowstone caldera, provides a mechanism for the precursory thinning of the crust required by a top down or “Plate” based tectonic model for the eastern Snake River Plain and Yellowstone volcanic system.